Implications of ABI stability#

Although Node-API provides an ABI stability guarantee, other parts of Node.js do not, and any external libraries used from the addon may not. In particular, none of the following APIs provide an ABI stability guarantee across major versions:

• the Node.js C++ APIs available via any of

  #include <node.h>
  #include <node_buffer.h>
  #include <node_version.h>
  #include <node_object_wrap.h>

• the libuv APIs which are also included with Node.js and available via

  #include <uv.h>

• the V8 API available via

  #include <v8.h>

Thus, for an addon to remain ABI-compatible across Node.js major versions, it must use Node-API exclusively by restricting itself to using

#include <node_api.h>

and by checking, for all external libraries that it uses, that the external library makes ABI stability guarantees similar to Node-API.

Building#

Unlike modules written in JavaScript, developing and deploying Node.js native addons using Node-API requires an additional set of tools. Besides the basic tools required to develop for Node.js, the native addon developer requires a toolchain that can compile C and C++ code into a binary. In addition, depending upon how the native addon is deployed, the user of the native addon will also need to have a C/C++ toolchain installed.

For Linux developers, the necessary C/C++ toolchain packages are readily available. GCC is widely used in the Node.js community to build and test across a variety of platforms. For many developers, the LLVM compiler infrastructure is also a good choice.

For Mac developers, Xcode offers all the required compiler tools. However, it is not necessary to install the entire Xcode IDE. The following command installs the necessary toolchain:

xcode-select --install

For Windows developers, Visual Studio offers all the required compiler tools. However, it is not necessary to install the entire Visual Studio IDE. The following command installs the necessary toolchain:

npm install --global windows-build-tools

The sections below describe the additional tools available for developing and deploying Node.js native addons.

Build tools#

Both the tools listed here require that users of the native addon have a C/C++ toolchain installed in order to successfully install the native addon.

node-gyp#

node-gyp is a build system based on Google's GYP tool and comes bundled with npm. GYP, and therefore node-gyp, requires that Python be installed.

Historically, node-gyp has been the tool of choice for building native addons. It has widespread adoption and documentation. However, some developers have run into limitations in node-gyp.

CMake.js#

CMake.js is an alternative build system based on CMake.

CMake.js is a good choice for projects that already use CMake or for developers affected by limitations in node-gyp.

Uploading precompiled binaries#

The three tools listed here permit native addon developers and maintainers to create and upload binaries to public or private servers. These tools are typically integrated with CI/CD build systems like Travis CI and AppVeyor to build and upload binaries for a variety of platforms and architectures. These binaries are then available for download by users who do not need to have a C/C++ toolchain installed.

node-pre-gyp#

node-pre-gyp is a tool based on node-gyp that adds the ability to upload binaries to a server of the developer's choice. node-pre-gyp has particularly good support for uploading binaries to Amazon S3.

prebuild#

prebuild is a tool that supports builds using either node-gyp or CMake.js. Unlike node-pre-gyp which supports a variety of servers, prebuild uploads binaries only to GitHub releases. prebuild is a good choice for GitHub projects using CMake.js.

prebuildify#

prebuildify is a tool based on node-gyp. The advantage of prebuildify is that the built binaries are bundled with the native module when it's uploaded to npm. The binaries are downloaded from npm and are immediately available to the module user when the native module is installed.

Usage#

In order to use the Node-API functions, include the file node_api.h which is located in the src directory in the node development tree:

#include <node_api.h>

This will opt into the default NAPI_VERSION for the given release of Node.js. In order to ensure compatibility with specific versions of Node-API, the version can be specified explicitly when including the header:

#define NAPI_VERSION 3
#include <node_api.h>

This restricts the Node-API surface to just the functionality that was available in the specified (and earlier) versions.

Some of the Node-API surface is experimental and requires explicit opt-in:

#define NAPI_EXPERIMENTAL
#include <node_api.h>

In this case the entire API surface, including any experimental APIs, will be available to the module code.

Node-API version matrix#

Node-API versions are additive and versioned independently from Node.js. Version 4 is an extension to version 3 in that it has all of the APIs from version 3 with some additions. This means that it is not necessary to recompile for new versions of Node.js which are listed as supporting a later version.

* Node-API was experimental.

** Node.js 8.0.0 included Node-API as experimental. It was released as Node-API version 1 but continued to evolve until Node.js 8.6.0. The API is different in versions prior to Node.js 8.6.0. We recommend Node-API version 3 or later.

Each API documented for Node-API will have a header named added in:, and APIs which are stable will have the additional header Node-API version:. APIs are directly usable when using a Node.js version which supports the Node-API version shown in Node-API version: or higher. When using a Node.js version that does not support the Node-API version: listed or if there is no Node-API version: listed, then the API will only be available if #define NAPI_EXPERIMENTAL precedes the inclusion of node_api.h or js_native_api.h. If an API appears not to be available on a version of Node.js which is later than the one shown in added in: then this is most likely the reason for the apparent absence.

The Node-APIs associated strictly with accessing ECMAScript features from native code can be found separately in js_native_api.h and js_native_api_types.h. The APIs defined in these headers are included in node_api.h and node_api_types.h. The headers are structured in this way in order to allow implementations of Node-API outside of Node.js. For those implementations the Node.js specific APIs may not be applicable.

The Node.js-specific parts of an addon can be separated from the code that exposes the actual functionality to the JavaScript environment so that the latter may be used with multiple implementations of Node-API. In the example below, addon.c and addon.h refer only to js_native_api.h. This ensures that addon.c can be reused to compile against either the Node.js implementation of Node-API or any implementation of Node-API outside of Node.js.

addon_node.c is a separate file that contains the Node.js specific entry point to the addon and which instantiates the addon by calling into addon.c when the addon is loaded into a Node.js environment.

// addon.h
#ifndef _ADDON_H_
#define _ADDON_H_
#include <js_native_api.h>
napi_value create_addon(napi_env env);
#endif  // _ADDON_H_

// addon.c
#include "addon.h"

#define NAPI_CALL(env, call)                                      \
  do {                                                            \
    napi_status status = (call);                                  \
    if (status != napi_ok) {                                      \
      const napi_extended_error_info* error_info = NULL;          \
      napi_get_last_error_info((env), &error_info);               \
      bool is_pending;                                            \
      napi_is_exception_pending((env), &is_pending);              \
      if (!is_pending) {                                          \
        const char* message = (error_info->error_message == NULL) \
            ? "empty error message"                               \
            : error_info->error_message;                          \
        napi_throw_error((env), NULL, message);                   \
        return NULL;                                              \
      }                                                           \
    }                                                             \
  } while(0)

static napi_value
DoSomethingUseful(napi_env env, napi_callback_info info) {
  // Do something useful.
  return NULL;
}

napi_value create_addon(napi_env env) {
  napi_value result;
  NAPI_CALL(env, napi_create_object(env, &result));

  napi_value exported_function;
  NAPI_CALL(env, napi_create_function(env,
                                      "doSomethingUseful",
                                      NAPI_AUTO_LENGTH,
                                      DoSomethingUseful,
                                      NULL,
                                      &exported_function));

  NAPI_CALL(env, napi_set_named_property(env,
                                         result,
                                         "doSomethingUseful",
                                         exported_function));

  return result;
}

// addon_node.c
#include <node_api.h>
#include "addon.h"

NAPI_MODULE_INIT() {
  // This function body is expected to return a `napi_value`.
  // The variables `napi_env env` and `napi_value exports` may be used within
  // the body, as they are provided by the definition of `NAPI_MODULE_INIT()`.
  return create_addon(env);
}

Environment life cycle APIs#

Section 8.7 of the ECMAScript Language Specification defines the concept of an "Agent" as a self-contained environment in which JavaScript code runs. Multiple such Agents may be started and terminated either concurrently or in sequence by the process.

A Node.js environment corresponds to an ECMAScript Agent. In the main process, an environment is created at startup, and additional environments can be created on separate threads to serve as worker threads. When Node.js is embedded in another application, the main thread of the application may also construct and destroy a Node.js environment multiple times during the life cycle of the application process such that each Node.js environment created by the application may, in turn, during its life cycle create and destroy additional environments as worker threads.

From the perspective of a native addon this means that the bindings it provides may be called multiple times, from multiple contexts, and even concurrently from multiple threads.

Native addons may need to allocate global state which they use during their entire life cycle such that the state must be unique to each instance of the addon.

To this end, Node-API provides a way to allocate data such that its life cycle is tied to the life cycle of the Agent.

napi_set_instance_data#

napi_status napi_set_instance_data(napi_env env,
                                   void* data,
                                   napi_finalize finalize_cb,
                                   void* finalize_hint);

• [in] env: The environment that the Node-API call is invoked under.
• [in] data: The data item to make available to bindings of this instance.
• [in] finalize_cb: The function to call when the environment is being torn down. The function receives data so that it might free it. napi_finalize provides more details.
• [in] finalize_hint: Optional hint to pass to the finalize callback during collection.

Returns napi_ok if the API succeeded.

This API associates data with the currently running Agent. data can later be retrieved using napi_get_instance_data(). Any existing data associated with the currently running Agent which was set by means of a previous call to napi_set_instance_data() will be overwritten. If a finalize_cb was provided by the previous call, it will not be called.

napi_get_instance_data#

napi_status napi_get_instance_data(napi_env env,
                                   void** data);

• [in] env: The environment that the Node-API call is invoked under.
• [out] data: The data item that was previously associated with the currently running Agent by a call to napi_set_instance_data().

Returns napi_ok if the API succeeded.

This API retrieves data that was previously associated with the currently running Agent via napi_set_instance_data(). If no data is set, the call will succeed and data will be set to NULL.

Basic Node-API data types#

Node-API exposes the following fundamental datatypes as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other Node-API calls.

napi_status#

Integral status code indicating the success or failure of a Node-API call. Currently, the following status codes are supported.

typedef enum {
  napi_ok,
  napi_invalid_arg,
  napi_object_expected,
  napi_string_expected,
  napi_name_expected,
  napi_function_expected,
  napi_number_expected,
  napi_boolean_expected,
  napi_array_expected,
  napi_generic_failure,
  napi_pending_exception,
  napi_cancelled,
  napi_escape_called_twice,
  napi_handle_scope_mismatch,
  napi_callback_scope_mismatch,
  napi_queue_full,
  napi_closing,
  napi_bigint_expected,
  napi_date_expected,
  napi_arraybuffer_expected,
  napi_detachable_arraybuffer_expected,
  napi_would_deadlock,  /* unused */
} napi_status;

If additional information is required upon an API returning a failed status, it can be obtained by calling napi_get_last_error_info.

napi_extended_error_info#

typedef struct {
  const char* error_message;
  void* engine_reserved;
  uint32_t engine_error_code;
  napi_status error_code;
} napi_extended_error_info;

• error_message: UTF8-encoded string containing a VM-neutral description of the error.
• engine_reserved: Reserved for VM-specific error details. This is currently not implemented for any VM.
• engine_error_code: VM-specific error code. This is currently not implemented for any VM.
• error_code: The Node-API status code that originated with the last error.

See the Error handling section for additional information.

napi_env#

napi_env is used to represent a context that the underlying Node-API implementation can use to persist VM-specific state. This structure is passed to native functions when they're invoked, and it must be passed back when making Node-API calls. Specifically, the same napi_env that was passed in when the initial native function was called must be passed to any subsequent nested Node-API calls. Caching the napi_env for the purpose of general reuse, and passing the napi_env between instances of the same addon running on different Worker threads is not allowed. The napi_env becomes invalid when an instance of a native addon is unloaded. Notification of this event is delivered through the callbacks given to napi_add_env_cleanup_hook and napi_set_instance_data.

napi_value#

This is an opaque pointer that is used to represent a JavaScript value.

napi_threadsafe_function#

This is an opaque pointer that represents a JavaScript function which can be called asynchronously from multiple threads via napi_call_threadsafe_function().

napi_threadsafe_function_release_mode#

A value to be given to napi_release_threadsafe_function() to indicate whether the thread-safe function is to be closed immediately (napi_tsfn_abort) or merely released (napi_tsfn_release) and thus available for subsequent use via napi_acquire_threadsafe_function() and napi_call_threadsafe_function().

typedef enum {
  napi_tsfn_release,
  napi_tsfn_abort
} napi_threadsafe_function_release_mode;

napi_threadsafe_function_call_mode#

A value to be given to napi_call_threadsafe_function() to indicate whether the call should block whenever the queue associated with the thread-safe function is full.

typedef enum {
  napi_tsfn_nonblocking,
  napi_tsfn_blocking
} napi_threadsafe_function_call_mode;

Node-API memory management types#

napi_handle_scope#

This is an abstraction used to control and modify the lifetime of objects created within a particular scope. In general, Node-API values are created within the context of a handle scope. When a native method is called from JavaScript, a default handle scope will exist. If the user does not explicitly create a new handle scope, Node-API values will be created in the default handle scope. For any invocations of code outside the execution of a native method (for instance, during a libuv callback invocation), the module is required to create a scope before invoking any functions that can result in the creation of JavaScript values.

Handle scopes are created using napi_open_handle_scope and are destroyed using napi_close_handle_scope. Closing the scope can indicate to the GC that all napi_values created during the lifetime of the handle scope are no longer referenced from the current stack frame.

For more details, review the Object lifetime management.

napi_escapable_handle_scope#

Escapable handle scopes are a special type of handle scope to return values created within a particular handle scope to a parent scope.

napi_ref#

This is the abstraction to use to reference a napi_value. This allows for users to manage the lifetimes of JavaScript values, including defining their minimum lifetimes explicitly.

For more details, review the Object lifetime management.

napi_type_tag#

A 128-bit value stored as two unsigned 64-bit integers. It serves as a UUID with which JavaScript objects can be "tagged" in order to ensure that they are of a certain type. This is a stronger check than napi_instanceof, because the latter can report a false positive if the object's prototype has been manipulated. Type-tagging is most useful in conjunction with napi_wrap because it ensures that the pointer retrieved from a wrapped object can be safely cast to the native type corresponding to the type tag that had been previously applied to the JavaScript object.

typedef struct {
  uint64_t lower;
  uint64_t upper;
} napi_type_tag;

napi_async_cleanup_hook_handle#

An opaque value returned by napi_add_async_cleanup_hook. It must be passed to napi_remove_async_cleanup_hook when the chain of asynchronous cleanup events completes.

Node-API callback types#

napi_callback_info#

Opaque datatype that is passed to a callback function. It can be used for getting additional information about the context in which the callback was invoked.

napi_callback#

Function pointer type for user-provided native functions which are to be exposed to JavaScript via Node-API. Callback functions should satisfy the following signature:

typedef napi_value (*napi_callback)(napi_env, napi_callback_info);

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside a napi_callback is not necessary.

napi_finalize#

Function pointer type for add-on provided functions that allow the user to be notified when externally-owned data is ready to be cleaned up because the object with which it was associated with, has been garbage-collected. The user must provide a function satisfying the following signature which would get called upon the object's collection. Currently, napi_finalize can be used for finding out when objects that have external data are collected.

typedef void (*napi_finalize)(napi_env env,
                              void* finalize_data,
                              void* finalize_hint);

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

napi_async_execute_callback#

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

typedef void (*napi_async_execute_callback)(napi_env env, void* data);

Implementations of this function must avoid making Node-API calls that execute JavaScript or interact with JavaScript objects. Node-API calls should be in the napi_async_complete_callback instead. Do not use the napi_env parameter as it will likely result in execution of JavaScript.

napi_async_complete_callback#

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

typedef void (*napi_async_complete_callback)(napi_env env,
                                             napi_status status,
                                             void* data);

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

napi_threadsafe_function_call_js#

Function pointer used with asynchronous thread-safe function calls. The callback will be called on the main thread. Its purpose is to use a data item arriving via the queue from one of the secondary threads to construct the parameters necessary for a call into JavaScript, usually via napi_call_function, and then make the call into JavaScript.

The data arriving from the secondary thread via the queue is given in the data parameter and the JavaScript function to call is given in the js_callback parameter.

Node-API sets up the environment prior to calling this callback, so it is sufficient to call the JavaScript function via napi_call_function rather than via napi_make_callback.

Callback functions must satisfy the following signature:

typedef void (*napi_threadsafe_function_call_js)(napi_env env,
                                                 napi_value js_callback,
                                                 void* context,
                                                 void* data);

• [in] env: The environment to use for API calls, or NULL if the thread-safe function is being torn down and data may need to be freed.
• [in] js_callback: The JavaScript function to call, or NULL if the thread-safe function is being torn down and data may need to be freed. It may also be NULL if the thread-safe function was created without js_callback.
• [in] context: The optional data with which the thread-safe function was created.
• [in] data: Data created by the secondary thread. It is the responsibility of the callback to convert this native data to JavaScript values (with Node-API functions) that can be passed as parameters when js_callback is invoked. This pointer is managed entirely by the threads and this callback. Thus this callback should free the data.

Unless for reasons discussed in Object Lifetime Management, creating a handle and/or callback scope inside the function body is not necessary.

napi_async_cleanup_hook#

Function pointer used with napi_add_async_cleanup_hook. It will be called when the environment is being torn down.

Callback functions must satisfy the following signature:

typedef void (*napi_async_cleanup_hook)(napi_async_cleanup_hook_handle handle,
                                        void* data);

• [in] handle: The handle that must be passed to napi_remove_async_cleanup_hook after completion of the asynchronous cleanup.
• [in] data: The data that was passed to napi_add_async_cleanup_hook.

The body of the function should initiate the asynchronous cleanup actions at the end of which handle must be passed in a call to napi_remove_async_cleanup_hook.

Error handling#

Node-API uses both return values and JavaScript exceptions for error handling. The following sections explain the approach for each case.

Return values#

All of the Node-API functions share the same error handling pattern. The return type of all API functions is napi_status.

The return value will be napi_ok if the request was successful and no uncaught JavaScript exception was thrown. If an error occurred AND an exception was thrown, the napi_status value for the error will be returned. If an exception was thrown, and no error occurred, napi_pending_exception will be returned.

In cases where a return value other than napi_ok or napi_pending_exception is returned, napi_is_exception_pending must be called to check if an exception is pending. See the section on exceptions for more details.

The full set of possible napi_status values is defined in napi_api_types.h.

The napi_status return value provides a VM-independent representation of the error which occurred. In some cases it is useful to be able to get more detailed information, including a string representing the error as well as VM (engine)-specific information.

In order to retrieve this information napi_get_last_error_info is provided which returns a napi_extended_error_info structure. The format of the napi_extended_error_info structure is as follows:

typedef struct napi_extended_error_info {
  const char* error_message;
  void* engine_reserved;
  uint32_t engine_error_code;
  napi_status error_code;
};

• error_message: Textual representation of the error that occurred.
• engine_reserved: Opaque handle reserved for engine use only.
• engine_error_code: VM specific error code.
• error_code: Node-API status code for the last error.

napi_get_last_error_info returns the information for the last Node-API call that was made.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

napi_get_last_error_info#

napi_status
napi_get_last_error_info(napi_env env,
                         const napi_extended_error_info** result);

• [in] env: The environment that the API is invoked under.
• [out] result: The napi_extended_error_info structure with more information about the error.

Returns napi_ok if the API succeeded.

This API retrieves a napi_extended_error_info structure with information about the last error that occurred.

The content of the napi_extended_error_info returned is only valid up until a Node-API function is called on the same env.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

This API can be called even if there is a pending JavaScript exception.

Exceptions#

Any Node-API function call may result in a pending JavaScript exception. This is the case for any of the API functions, even those that may not cause the execution of JavaScript.

If the napi_status returned by a function is napi_ok then no exception is pending and no additional action is required. If the napi_status returned is anything other than napi_ok or napi_pending_exception, in order to try to recover and continue instead of simply returning immediately, napi_is_exception_pending must be called in order to determine if an exception is pending or not.

In many cases when a Node-API function is called and an exception is already pending, the function will return immediately with a napi_status of napi_pending_exception. However, this is not the case for all functions. Node-API allows a subset of the functions to be called to allow for some minimal cleanup before returning to JavaScript. In that case, napi_status will reflect the status for the function. It will not reflect previous pending exceptions. To avoid confusion, check the error status after every function call.

When an exception is pending one of two approaches can be employed.

The first approach is to do any appropriate cleanup and then return so that execution will return to JavaScript. As part of the transition back to JavaScript, the exception will be thrown at the point in the JavaScript code where the native method was invoked. The behavior of most Node-API calls is unspecified while an exception is pending, and many will simply return napi_pending_exception, so do as little as possible and then return to JavaScript where the exception can be handled.

The second approach is to try to handle the exception. There will be cases where the native code can catch the exception, take the appropriate action, and then continue. This is only recommended in specific cases where it is known that the exception can be safely handled. In these cases napi_get_and_clear_last_exception can be used to get and clear the exception. On success, result will contain the handle to the last JavaScript Object thrown. If it is determined, after retrieving the exception, the exception cannot be handled after all it can be re-thrown it with napi_throw where error is the JavaScript Error object to be thrown.

The following utility functions are also available in case native code needs to throw an exception or determine if a napi_value is an instance of a JavaScript Error object: napi_throw_error, napi_throw_type_error, napi_throw_range_error and napi_is_error.

The following utility functions are also available in case native code needs to create an Error object: napi_create_error, napi_create_type_error, and napi_create_range_error, where result is the napi_value that refers to the newly created JavaScript Error object.

The Node.js project is adding error codes to all of the errors generated internally. The goal is for applications to use these error codes for all error checking. The associated error messages will remain, but will only be meant to be used for logging and display with the expectation that the message can change without SemVer applying. In order to support this model with Node-API, both in internal functionality and for module specific functionality (as its good practice), the throw_ and create_ functions take an optional code parameter which is the string for the code to be added to the error object. If the optional parameter is NULL then no code will be associated with the error. If a code is provided, the name associated with the error is also updated to be:

originalName [code]

where originalName is the original name associated with the error and code is the code that was provided. For example, if the code is 'ERR_ERROR_1' and a TypeError is being created the name will be:

TypeError [ERR_ERROR_1]

napi_throw#

NAPI_EXTERN napi_status napi_throw(napi_env env, napi_value error);

• [in] env: The environment that the API is invoked under.
• [in] error: The JavaScript value to be thrown.

Returns napi_ok if the API succeeded.

This API throws the JavaScript value provided.

napi_throw_error#

NAPI_EXTERN napi_status napi_throw_error(napi_env env,
                                         const char* code,
                                         const char* msg);

• [in] env: The environment that the API is invoked under.
• [in] code: Optional error code to be set on the error.
• [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript Error with the text provided.

napi_throw_type_error#

NAPI_EXTERN napi_status napi_throw_type_error(napi_env env,
                                              const char* code,
                                              const char* msg);

• [in] env: The environment that the API is invoked under.
• [in] code: Optional error code to be set on the error.
• [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript TypeError with the text provided.

napi_throw_range_error#

NAPI_EXTERN napi_status napi_throw_range_error(napi_env env,
                                               const char* code,
                                               const char* msg);

• [in] env: The environment that the API is invoked under.
• [in] code: Optional error code to be set on the error.
• [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript RangeError with the text provided.

napi_is_error#

NAPI_EXTERN napi_status napi_is_error(napi_env env,
                                      napi_value value,
                                      bool* result);

• [in] env: The environment that the API is invoked under.
• [in] value: The napi_value to be checked.
• [out] result: Boolean value that is set to true if napi_value represents an error, false otherwise.

Returns napi_ok if the API succeeded.

This API queries a napi_value to check if it represents an error object.

napi_create_error#

NAPI_EXTERN napi_status napi_create_error(napi_env env,
                                          napi_value code,
                                          napi_value msg,
                                          napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] code: Optional napi_value with the string for the error code to be associated with the error.
• [in] msg: napi_value that references a JavaScript String to be used as the message for the Error.
• [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript Error with the text provided.

napi_create_type_error#

NAPI_EXTERN napi_status napi_create_type_error(napi_env env,
                                               napi_value code,
                                               napi_value msg,
                                               napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] code: Optional napi_value with the string for the error code to be associated with the error.
• [in] msg: napi_value that references a JavaScript String to be used as the message for the Error.
• [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript TypeError with the text provided.

napi_create_range_error#

NAPI_EXTERN napi_status napi_create_range_error(napi_env env,
                                                napi_value code,
                                                napi_value msg,
                                                napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] code: Optional napi_value with the string for the error code to be associated with the error.
• [in] msg: napi_value that references a JavaScript String to be used as the message for the Error.
• [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript RangeError with the text provided.

napi_get_and_clear_last_exception#

napi_status napi_get_and_clear_last_exception(napi_env env,
                                              napi_value* result);

• [in] env: The environment that the API is invoked under.
• [out] result: The exception if one is pending, NULL otherwise.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_is_exception_pending#

napi_status napi_is_exception_pending(napi_env env, bool* result);

• [in] env: The environment that the API is invoked under.
• [out] result: Boolean value that is set to true if an exception is pending.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_fatal_exception#

napi_status napi_fatal_exception(napi_env env, napi_value err);

• [in] env: The environment that the API is invoked under.
• [in] err: The error that is passed to 'uncaughtException'.

Trigger an 'uncaughtException' in JavaScript. Useful if an async callback throws an exception with no way to recover.

Fatal errors#

In the event of an unrecoverable error in a native module, a fatal error can be thrown to immediately terminate the process.

napi_fatal_error#

NAPI_NO_RETURN void napi_fatal_error(const char* location,
                                     size_t location_len,
                                     const char* message,
                                     size_t message_len);

• [in] location: Optional location at which the error occurred.
• [in] location_len: The length of the location in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
• [in] message: The message associated with the error.
• [in] message_len: The length of the message in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.

The function call does not return, the process will be terminated.

This API can be called even if there is a pending JavaScript exception.

Object lifetime management#

As Node-API calls are made, handles to objects in the heap for the underlying VM may be returned as napi_values. These handles must hold the objects 'live' until they are no longer required by the native code, otherwise the objects could be collected before the native code was finished using them.

As object handles are returned they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call.

In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the Node-API functions that can be used to change the handle lifespan from the default.

Making handle lifespan shorter than that of the native method#

It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which iterates through the elements in a large array:

for (int i = 0; i < 1000000; i++) {
  napi_value result;
  napi_status status = napi_get_element(env, object, i, &result);
  if (status != napi_ok) {
    break;
  }
  // do something with element
}

This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recent handle, all of the associated objects would also be kept alive since they all share the same scope.

To handle this case, Node-API provides the ability to establish a new 'scope' to which newly created handles will be associated. Once those handles are no longer required, the scope can be 'closed' and any handles associated with the scope are invalidated. The methods available to open/close scopes are napi_open_handle_scope and napi_close_handle_scope.

Node-API only supports a single nested hierarchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be closed in the reverse order from which they are opened. In addition, all scopes created within a native method must be closed before returning from that method.

Taking the earlier example, adding calls to napi_open_handle_scope and napi_close_handle_scope would ensure that at most a single handle is valid throughout the execution of the loop:

for (int i = 0; i < 1000000; i++) {
  napi_handle_scope scope;
  napi_status status = napi_open_handle_scope(env, &scope);
  if (status != napi_ok) {
    break;
  }
  napi_value result;
  status = napi_get_element(env, object, i, &result);
  if (status != napi_ok) {
    break;
  }
  // do something with element
  status = napi_close_handle_scope(env, scope);
  if (status != napi_ok) {
    break;
  }
}

When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. Node-API supports an 'escapable scope' in order to support this case. An escapable scope allows one handle to be 'promoted' so that it 'escapes' the current scope and the lifespan of the handle changes from the current scope to that of the outer scope.

The methods available to open/close escapable scopes are napi_open_escapable_handle_scope and napi_close_escapable_handle_scope.

The request to promote a handle is made through napi_escape_handle which can only be called once.

napi_open_handle_scope#

NAPI_EXTERN napi_status napi_open_handle_scope(napi_env env,
                                               napi_handle_scope* result);

• [in] env: The environment that the API is invoked under.
• [out] result: napi_value representing the new scope.

Returns napi_ok if the API succeeded.

This API opens a new scope.

napi_close_handle_scope#

NAPI_EXTERN napi_status napi_close_handle_scope(napi_env env,
                                                napi_handle_scope scope);

• [in] env: The environment that the API is invoked under.
• [in] scope: napi_value representing the scope to be closed.

Returns napi_ok if the API succeeded.

This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.

This API can be called even if there is a pending JavaScript exception.

napi_open_escapable_handle_scope#

NAPI_EXTERN napi_status
    napi_open_escapable_handle_scope(napi_env env,
                                     napi_handle_scope* result);

• [in] env: The environment that the API is invoked under.
• [out] result: napi_value representing the new scope.

Returns napi_ok if the API succeeded.

This API opens a new scope from which one object can be promoted to the outer scope.

napi_close_escapable_handle_scope#

NAPI_EXTERN napi_status
    napi_close_escapable_handle_scope(napi_env env,
                                      napi_handle_scope scope);

• [in] env: The environment that the API is invoked under.
• [in] scope: napi_value representing the scope to be closed.

Returns napi_ok if the API succeeded.

This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.

This API can be called even if there is a pending JavaScript exception.

napi_escape_handle#

napi_status napi_escape_handle(napi_env env,
                               napi_escapable_handle_scope scope,
                               napi_value escapee,
                               napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] scope: napi_value representing the current scope.
• [in] escapee: napi_value representing the JavaScript Object to be escaped.
• [out] result: napi_value representing the handle to the escaped Object in the outer scope.

Returns napi_ok if the API succeeded.

This API promotes the handle to the JavaScript object so that it is valid for the lifetime of the outer scope. It can only be called once per scope. If it is called more than once an error will be returned.

This API can be called even if there is a pending JavaScript exception.

References to objects with a lifespan longer than that of the native method#

In some cases an addon will need to be able to create and reference objects with a lifespan longer than that of a single native method invocation. For example, to create a constructor and later use that constructor in a request to creates instances, it must be possible to reference the constructor object across many different instance creation requests. This would not be possible with a normal handle returned as a napi_value as described in the earlier section. The lifespan of a normal handle is managed by scopes and all scopes must be closed before the end of a native method.

Node-API provides methods to create persistent references to an object. Each persistent reference has an associated count with a value of 0 or higher. The count determines if the reference will keep the corresponding object live. References with a count of 0 do not prevent the object from being collected and are often called 'weak' references. Any count greater than 0 will prevent the object from being collected.

References can be created with an initial reference count. The count can then be modified through napi_reference_ref and napi_reference_unref. If an object is collected while the count for a reference is 0, all subsequent calls to get the object associated with the reference napi_get_reference_value will return NULL for the returned napi_value. An attempt to call napi_reference_ref for a reference whose object has been collected results in an error.

References must be deleted once they are no longer required by the addon. When a reference is deleted, it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference results in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever.

There can be multiple persistent references created which refer to the same object, each of which will either keep the object live or not based on its individual count.

napi_create_reference#

NAPI_EXTERN napi_status napi_create_reference(napi_env env,
                                              napi_value value,
                                              uint32_t initial_refcount,
                                              napi_ref* result);

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing the Object to which we want a reference.
• [in] initial_refcount: Initial reference count for the new reference.
• [out] result: napi_ref pointing to the new reference.

Returns napi_ok if the API succeeded.

This API create a new reference with the specified reference count to the Object passed in.

napi_delete_reference#

NAPI_EXTERN napi_status napi_delete_reference(napi_env env, napi_ref ref);

• [in] env: The environment that the API is invoked under.
• [in] ref: napi_ref to be deleted.

Returns napi_ok if the API succeeded.

This API deletes the reference passed in.

This API can be called even if there is a pending JavaScript exception.

napi_reference_ref#

NAPI_EXTERN napi_status napi_reference_ref(napi_env env,
                                           napi_ref ref,
                                           uint32_t* result);

• [in] env: The environment that the API is invoked under.
• [in] ref: napi_ref for which the reference count will be incremented.
• [out] result: The new reference count.

Returns napi_ok if the API succeeded.

This API increments the reference count for the reference passed in and returns the resulting reference count.

napi_reference_unref#

NAPI_EXTERN napi_status napi_reference_unref(napi_env env,
                                             napi_ref ref,
                                             uint32_t* result);

• [in] env: The environment that the API is invoked under.
• [in] ref: napi_ref for which the reference count will be decremented.
• [out] result: The new reference count.

Returns napi_ok if the API succeeded.

This API decrements the reference count for the reference passed in and returns the resulting reference count.

napi_get_reference_value#

NAPI_EXTERN napi_status napi_get_reference_value(napi_env env,
                                                 napi_ref ref,
                                                 napi_value* result);

the napi_value passed in or out of these methods is a handle to the object to which the reference is related.

• [in] env: The environment that the API is invoked under.
• [in] ref: napi_ref for which we requesting the corresponding Object.
• [out] result: The napi_value for the Object referenced by the napi_ref.

Returns napi_ok if the API succeeded.

If still valid, this API returns the napi_value representing the JavaScript Object associated with the napi_ref. Otherwise, result will be NULL.

Cleanup on exit of the current Node.js instance#

While a Node.js process typically releases all its resources when exiting, embedders of Node.js, or future Worker support, may require addons to register clean-up hooks that will be run once the current Node.js instance exits.

Node-API provides functions for registering and un-registering such callbacks. When those callbacks are run, all resources that are being held by the addon should be freed up.

napi_add_env_cleanup_hook#

NODE_EXTERN napi_status napi_add_env_cleanup_hook(napi_env env,
                                                  void (*fun)(void* arg),
                                                  void* arg);

Registers fun as a function to be run with the arg parameter once the current Node.js environment exits.

A function can safely be specified multiple times with different arg values. In that case, it will be called multiple times as well. Providing the same fun and arg values multiple times is not allowed and will lead the process to abort.

The hooks will be called in reverse order, i.e. the most recently added one will be called first.

Removing this hook can be done by using napi_remove_env_cleanup_hook. Typically, that happens when the resource for which this hook was added is being torn down anyway.

For asynchronous cleanup, napi_add_async_cleanup_hook is available.

napi_remove_env_cleanup_hook#

NAPI_EXTERN napi_status napi_remove_env_cleanup_hook(napi_env env,
                                                     void (*fun)(void* arg),
                                                     void* arg);

Unregisters fun as a function to be run with the arg parameter once the current Node.js environment exits. Both the argument and the function value need to be exact matches.

The function must have originally been registered with napi_add_env_cleanup_hook, otherwise the process will abort.

napi_add_async_cleanup_hook#

NAPI_EXTERN napi_status napi_add_async_cleanup_hook(
    napi_env env,
    napi_async_cleanup_hook hook,
    void* arg,
    napi_async_cleanup_hook_handle* remove_handle);

• [in] env: The environment that the API is invoked under.
• [in] hook: The function pointer to call at environment teardown.
• [in] arg: The pointer to pass to hook when it gets called.
• [out] remove_handle: Optional handle that refers to the asynchronous cleanup hook.

Registers hook, which is a function of type napi_async_cleanup_hook, as a function to be run with the remove_handle and arg parameters once the current Node.js environment exits.

Unlike napi_add_env_cleanup_hook, the hook is allowed to be asynchronous.

Otherwise, behavior generally matches that of napi_add_env_cleanup_hook.

If remove_handle is not NULL, an opaque value will be stored in it that must later be passed to napi_remove_async_cleanup_hook, regardless of whether the hook has already been invoked. Typically, that happens when the resource for which this hook was added is being torn down anyway.

napi_remove_async_cleanup_hook#

NAPI_EXTERN napi_status napi_remove_async_cleanup_hook(
    napi_async_cleanup_hook_handle remove_handle);

• [in] remove_handle: The handle to an asynchronous cleanup hook that was created with napi_add_async_cleanup_hook.

Unregisters the cleanup hook corresponding to remove_handle. This will prevent the hook from being executed, unless it has already started executing. This must be called on any napi_async_cleanup_hook_handle value obtained from napi_add_async_cleanup_hook.

Module registration#

Node-API modules are registered in a manner similar to other modules except that instead of using the NODE_MODULE macro the following is used:

NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)

The next difference is the signature for the Init method. For a Node-API module it is as follows:

napi_value Init(napi_env env, napi_value exports);

The return value from Init is treated as the exports object for the module. The Init method is passed an empty object via the exports parameter as a convenience. If Init returns NULL, the parameter passed as exports is exported by the module. Node-API modules cannot modify the module object but can specify anything as the exports property of the module.

To add the method hello as a function so that it can be called as a method provided by the addon:

napi_value Init(napi_env env, napi_value exports) {
  napi_status status;
  napi_property_descriptor desc = {
    "hello",
    NULL,
    Method,
    NULL,
    NULL,
    NULL,
    napi_writable | napi_enumerable | napi_configurable,
    NULL
  };
  status = napi_define_properties(env, exports, 1, &desc);
  if (status != napi_ok) return NULL;
  return exports;
}

To set a function to be returned by the require() for the addon:

napi_value Init(napi_env env, napi_value exports) {
  napi_value method;
  napi_status status;
  status = napi_create_function(env, "exports", NAPI_AUTO_LENGTH, Method, NULL, &method);
  if (status != napi_ok) return NULL;
  return method;
}

To define a class so that new instances can be created (often used with Object wrap):

// NOTE: partial example, not all referenced code is included
napi_value Init(napi_env env, napi_value exports) {
  napi_status status;
  napi_property_descriptor properties[] = {
    { "value", NULL, NULL, GetValue, SetValue, NULL, napi_writable | napi_configurable, NULL },
    DECLARE_NAPI_METHOD("plusOne", PlusOne),
    DECLARE_NAPI_METHOD("multiply", Multiply),
  };

  napi_value cons;
  status =
      napi_define_class(env, "MyObject", New, NULL, 3, properties, &cons);
  if (status != napi_ok) return NULL;

  status = napi_create_reference(env, cons, 1, &constructor);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "MyObject", cons);
  if (status != napi_ok) return NULL;

  return exports;
}

You can also use the NAPI_MODULE_INIT macro, which acts as a shorthand for NAPI_MODULE and defining an Init function:

NAPI_MODULE_INIT() {
  napi_value answer;
  napi_status result;

  status = napi_create_int64(env, 42, &answer);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "answer", answer);
  if (status != napi_ok) return NULL;

  return exports;
}

All Node-API addons are context-aware, meaning they may be loaded multiple times. There are a few design considerations when declaring such a module. The documentation on context-aware addons provides more details.

The variables env and exports will be available inside the function body following the macro invocation.

For more details on setting properties on objects, see the section on Working with JavaScript properties.

For more details on building addon modules in general, refer to the existing API.

Working with JavaScript values#

Node-API exposes a set of APIs to create all types of JavaScript values. Some of these types are documented under Section 6 of the ECMAScript Language Specification.

Fundamentally, these APIs are used to do one of the following:

1. Create a new JavaScript object
2. Convert from a primitive C type to a Node-API value
3. Convert from Node-API value to a primitive C type
4. Get global instances including undefined and null

Node-API values are represented by the type napi_value. Any Node-API call that requires a JavaScript value takes in a napi_value. In some cases, the API does check the type of the napi_value up-front. However, for better performance, it's better for the caller to make sure that the napi_value in question is of the JavaScript type expected by the API.

Enum types#

napi_key_collection_mode#

typedef enum {
  napi_key_include_prototypes,
  napi_key_own_only
} napi_key_collection_mode;

Describes the Keys/Properties filter enums:

napi_key_collection_mode limits the range of collected properties.

napi_key_own_only limits the collected properties to the given object only. napi_key_include_prototypes will include all keys of the objects's prototype chain as well.

napi_key_filter#

typedef enum {
  napi_key_all_properties = 0,
  napi_key_writable = 1,
  napi_key_enumerable = 1 << 1,
  napi_key_configurable = 1 << 2,
  napi_key_skip_strings = 1 << 3,
  napi_key_skip_symbols = 1 << 4
} napi_key_filter;

Property filter bits. They can be or'ed to build a composite filter.

napi_key_conversion#

typedef enum {
  napi_key_keep_numbers,
  napi_key_numbers_to_strings
} napi_key_conversion;

napi_key_numbers_to_strings will convert integer indices to strings. napi_key_keep_numbers will return numbers for integer indices.

napi_valuetype#

typedef enum {
  // ES6 types (corresponds to typeof)
  napi_undefined,
  napi_null,
  napi_boolean,
  napi_number,
  napi_string,
  napi_symbol,
  napi_object,
  napi_function,
  napi_external,
  napi_bigint,
} napi_valuetype;

Describes the type of a napi_value. This generally corresponds to the types described in Section 6.1 of the ECMAScript Language Specification. In addition to types in that section, napi_valuetype can also represent Functions and Objects with external data.

A JavaScript value of type napi_external appears in JavaScript as a plain object such that no properties can be set on it, and no prototype.

napi_typedarray_type#

typedef enum {
  napi_int8_array,
  napi_uint8_array,
  napi_uint8_clamped_array,
  napi_int16_array,
  napi_uint16_array,
  napi_int32_array,
  napi_uint32_array,
  napi_float32_array,
  napi_float64_array,
  napi_bigint64_array,
  napi_biguint64_array,
} napi_typedarray_type;

This represents the underlying binary scalar datatype of the TypedArray. Elements of this enum correspond to Section 22.2 of the ECMAScript Language Specification.

Object creation functions#

napi_create_array#

napi_status napi_create_array(napi_env env, napi_value* result)

• [in] env: The environment that the Node-API call is invoked under.
• [out] result: A napi_value representing a JavaScript Array.

Returns napi_ok if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript Array type. JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.

napi_create_array_with_length#

napi_status napi_create_array_with_length(napi_env env,
                                          size_t length,
                                          napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] length: The initial length of the Array.
• [out] result: A napi_value representing a JavaScript Array.

Returns napi_ok if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript Array type. The Array's length property is set to the passed-in length parameter. However, the underlying buffer is not guaranteed to be pre-allocated by the VM when the array is created. That behavior is left to the underlying VM implementation. If the buffer must be a contiguous block of memory that can be directly read and/or written via C, consider using napi_create_external_arraybuffer.

JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.

napi_create_arraybuffer#

napi_status napi_create_arraybuffer(napi_env env,
                                    size_t byte_length,
                                    void** data,
                                    napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] length: The length in bytes of the array buffer to create.
• [out] data: Pointer to the underlying byte buffer of the ArrayBuffer.
• [out] result: A napi_value representing a JavaScript ArrayBuffer.

Returns napi_ok if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript ArrayBuffer. ArrayBuffers are used to represent fixed-length binary data buffers. They are normally used as a backing-buffer for TypedArray objects. The ArrayBuffer allocated will have an underlying byte buffer whose size is determined by the length parameter that's passed in. The underlying buffer is optionally returned back to the caller in case the caller wants to directly manipulate the buffer. This buffer can only be written to directly from native code. To write to this buffer from JavaScript, a typed array or DataView object would need to be created.

JavaScript ArrayBuffer objects are described in Section 24.1 of the ECMAScript Language Specification.

napi_create_buffer#

napi_status napi_create_buffer(napi_env env,
                               size_t size,
                               void** data,
                               napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] size: Size in bytes of the underlying buffer.
• [out] data: Raw pointer to the underlying buffer.
• [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

This API allocates a node::Buffer object. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

napi_create_buffer_copy#

napi_status napi_create_buffer_copy(napi_env env,
                                    size_t length,
                                    const void* data,
                                    void** result_data,
                                    napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] size: Size in bytes of the input buffer (should be the same as the size of the new buffer).
• [in] data: Raw pointer to the underlying buffer to copy from.
• [out] result_data: Pointer to the new Buffer's underlying data buffer.
• [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

This API allocates a node::Buffer object and initializes it with data copied from the passed-in buffer. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

napi_create_date#

napi_status napi_create_date(napi_env env,
                             double time,
                             napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] time: ECMAScript time value in milliseconds since 01 January, 1970 UTC.
• [out] result: A napi_value representing a JavaScript Date.

Returns napi_ok if the API succeeded.

This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.

This API allocates a JavaScript Date object.

JavaScript Date objects are described in Section 20.3 of the ECMAScript Language Specification.

napi_create_external#

napi_status napi_create_external(napi_env env,
                                 void* data,
                                 napi_finalize finalize_cb,
                                 void* finalize_hint,
                                 napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] data: Raw pointer to the external data.
• [in] finalize_cb: Optional callback to call when the external value is being collected. napi_finalize provides more details.
• [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
• [out] result: A napi_value representing an external value.

Returns napi_ok if the API succeeded.

This API allocates a JavaScript value with external data attached to it. This is used to pass external data through JavaScript code, so it can be retrieved later by native code using napi_get_value_external.

The API adds a napi_finalize callback which will be called when the JavaScript object just created is ready for garbage collection. It is similar to napi_wrap() except that:

• the native data cannot be retrieved later using napi_unwrap(),
• nor can it be removed later using napi_remove_wrap(), and
• the object created by the API can be used with napi_wrap().

The created value is not an object, and therefore does not support additional properties. It is considered a distinct value type: calling napi_typeof() with an external value yields napi_external.

napi_create_external_arraybuffer#

napi_status
napi_create_external_arraybuffer(napi_env env,
                                 void* external_data,
                                 size_t byte_length,
                                 napi_finalize finalize_cb,
                                 void* finalize_hint,
                                 napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] external_data: Pointer to the underlying byte buffer of the ArrayBuffer.
• [in] byte_length: The length in bytes of the underlying buffer.
• [in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected. napi_finalize provides more details.
• [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
• [out] result: A napi_value representing a JavaScript ArrayBuffer.

Returns napi_ok if the API succeeded.

This API returns a Node-API value corresponding to a JavaScript ArrayBuffer. The underlying byte buffer of the ArrayBuffer is externally allocated and managed. The caller must ensure that the byte buffer remains valid until the finalize callback is called.

The API adds a napi_finalize callback which will be called when the JavaScript object just created is ready for garbage collection. It is similar to napi_wrap() except that:

• the native data cannot be retrieved later using napi_unwrap(),
• nor can it be removed later using napi_remove_wrap(), and
• the object created by the API can be used with napi_wrap().

JavaScript ArrayBuffers are described in Section 24.1 of the ECMAScript Language Specification.

napi_create_external_buffer#

napi_status napi_create_external_buffer(napi_env env,
                                        size_t length,
                                        void* data,
                                        napi_finalize finalize_cb,
                                        void* finalize_hint,
                                        napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] length: Size in bytes of the input buffer (should be the same as the size of the new buffer).
• [in] data: Raw pointer to the underlying buffer to expose to JavaScript.
• [in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected. napi_finalize provides more details.
• [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
• [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

This API allocates a node::Buffer object and initializes it with data backed by the passed in buffer. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

The API adds a napi_finalize callback which will be called when the JavaScript object just created is ready for garbage collection. It is similar to napi_wrap() except that:

• the native data cannot be retrieved later using napi_unwrap(),
• nor can it be removed later using napi_remove_wrap(), and
• the object created by the API can be used with napi_wrap().

For Node.js >=4 Buffers are Uint8Arrays.

napi_create_object#

napi_status napi_create_object(napi_env env, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [out] result: A napi_value representing a JavaScript Object.

Returns napi_ok if the API succeeded.

This API allocates a default JavaScript Object. It is the equivalent of doing new Object() in JavaScript.

The JavaScript Object type is described in Section 6.1.7 of the ECMAScript Language Specification.

napi_create_symbol#

napi_status napi_create_symbol(napi_env env,
                               napi_value description,
                               napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] description: Optional napi_value which refers to a JavaScript String to be set as the description for the symbol.
• [out] result: A napi_value representing a JavaScript Symbol.

Returns napi_ok if the API succeeded.

This API creates a JavaScript Symbol object from a UTF8-encoded C string.

The JavaScript Symbol type is described in Section 19.4 of the ECMAScript Language Specification.

napi_create_typedarray#

napi_status napi_create_typedarray(napi_env env,
                                   napi_typedarray_type type,
                                   size_t length,
                                   napi_value arraybuffer,
                                   size_t byte_offset,
                                   napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] type: Scalar datatype of the elements within the TypedArray.
• [in] length: Number of elements in the TypedArray.
• [in] arraybuffer: ArrayBuffer underlying the typed array.
• [in] byte_offset: The byte offset within the ArrayBuffer from which to start projecting the TypedArray.
• [out] result: A napi_value representing a JavaScript TypedArray.

Returns napi_ok if the API succeeded.

This API creates a JavaScript TypedArray object over an existing ArrayBuffer. TypedArray objects provide an array-like view over an underlying data buffer where each element has the same underlying binary scalar datatype.

It's required that (length * size_of_element) + byte_offset should be <= the size in bytes of the array passed in. If not, a RangeError exception is raised.

JavaScript TypedArray objects are described in Section 22.2 of the ECMAScript Language Specification.

napi_create_dataview#

napi_status napi_create_dataview(napi_env env,
                                 size_t byte_length,
                                 napi_value arraybuffer,
                                 size_t byte_offset,
                                 napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] length: Number of elements in the DataView.
• [in] arraybuffer: ArrayBuffer underlying the DataView.
• [in] byte_offset: The byte offset within the ArrayBuffer from which to start projecting the DataView.
• [out] result: A napi_value representing a JavaScript DataView.

Returns napi_ok if the API succeeded.

This API creates a JavaScript DataView object over an existing ArrayBuffer. DataView objects provide an array-like view over an underlying data buffer, but one which allows items of different size and type in the ArrayBuffer.

It is required that byte_length + byte_offset is less than or equal to the size in bytes of the array passed in. If not, a RangeError exception is raised.

JavaScript DataView objects are described in Section 24.3 of the ECMAScript Language Specification.

Functions to convert from C types to Node-API#

napi_create_int32#

napi_status napi_create_int32(napi_env env, int32_t value, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: Integer value to be represented in JavaScript.
• [out] result: A napi_value representing a JavaScript Number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C int32_t type to the JavaScript Number type.

The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_uint32#

napi_status napi_create_uint32(napi_env env, uint32_t value, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: Unsigned integer value to be represented in JavaScript.
• [out] result: A napi_value representing a JavaScript Number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C uint32_t type to the JavaScript Number type.

The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_int64#

napi_status napi_create_int64(napi_env env, int64_t value, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: Integer value to be represented in JavaScript.
• [out] result: A napi_value representing a JavaScript Number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C int64_t type to the JavaScript Number type.

The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification. Note the complete range of int64_t cannot be represented with full precision in JavaScript. Integer values outside the range of Number.MIN_SAFE_INTEGER -(2**53 - 1) - Number.MAX_SAFE_INTEGER (2**53 - 1) will lose precision.

napi_create_double#

napi_status napi_create_double(napi_env env, double value, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: Double-precision value to be represented in JavaScript.
• [out] result: A napi_value representing a JavaScript Number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C double type to the JavaScript Number type.

The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_bigint_int64#

napi_status napi_create_bigint_int64(napi_env env,
                                     int64_t value,
                                     napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] value: Integer value to be represented in JavaScript.
• [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts the C int64_t type to the JavaScript BigInt type.

napi_create_bigint_uint64#

napi_status napi_create_bigint_uint64(napi_env env,
                                      uint64_t value,
                                      napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] value: Unsigned integer value to be represented in JavaScript.
• [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts the C uint64_t type to the JavaScript BigInt type.

napi_create_bigint_words#

napi_status napi_create_bigint_words(napi_env env,
                                     int sign_bit,
                                     size_t word_count,
                                     const uint64_t* words,
                                     napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] sign_bit: Determines if the resulting BigInt will be positive or negative.
• [in] word_count: The length of the words array.
• [in] words: An array of uint64_t little-endian 64-bit words.
• [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts an array of unsigned 64-bit words into a single BigInt value.

The resulting BigInt is calculated as: (–1)^sign_bit (words[0] × (2⁶⁴)⁰ + words[1] × (2⁶⁴)¹ + …)

napi_create_string_latin1#

napi_status napi_create_string_latin1(napi_env env,
                                      const char* str,
                                      size_t length,
                                      napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] str: Character buffer representing an ISO-8859-1-encoded string.
• [in] length: The length of the string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
• [out] result: A napi_value representing a JavaScript String.

Returns napi_ok if the API succeeded.

This API creates a JavaScript String object from an ISO-8859-1-encoded C string. The native string is copied.

The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.

napi_create_string_utf16#

napi_status napi_create_string_utf16(napi_env env,
                                     const char16_t* str,
                                     size_t length,
                                     napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] str: Character buffer representing a UTF16-LE-encoded string.
• [in] length: The length of the string in two-byte code units, or NAPI_AUTO_LENGTH if it is null-terminated.
• [out] result: A napi_value representing a JavaScript String.

Returns napi_ok if the API succeeded.

This API creates a JavaScript String object from a UTF16-LE-encoded C string. The native string is copied.

The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.

napi_create_string_utf8#

napi_status napi_create_string_utf8(napi_env env,
                                    const char* str,
                                    size_t length,
                                    napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] str: Character buffer representing a UTF8-encoded string.
• [in] length: The length of the string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
• [out] result: A napi_value representing a JavaScript String.

Returns napi_ok if the API succeeded.

This API creates a JavaScript String object from a UTF8-encoded C string. The native string is copied.

The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.

Functions to convert from Node-API to C types#

napi_get_array_length#

napi_status napi_get_array_length(napi_env env,
                                  napi_value value,
                                  uint32_t* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing the JavaScript Array whose length is being queried.
• [out] result: uint32 representing length of the array.

Returns napi_ok if the API succeeded.

This API returns the length of an array.

Array length is described in Section 22.1.4.1 of the ECMAScript Language Specification.

napi_get_arraybuffer_info#

napi_status napi_get_arraybuffer_info(napi_env env,
                                      napi_value arraybuffer,
                                      void** data,
                                      size_t* byte_length)

• [in] env: The environment that the API is invoked under.
• [in] arraybuffer: napi_value representing the ArrayBuffer being queried.
• [out] data: The underlying data buffer of the ArrayBuffer. If byte_length is 0, this may be NULL or any other pointer value.
• [out] byte_length: Length in bytes of the underlying data buffer.

Returns napi_ok if the API succeeded.

This API is used to retrieve the underlying data buffer of an ArrayBuffer and its length.

WARNING: Use caution while using this API. The lifetime of the underlying data buffer is managed by the ArrayBuffer even after it's returned. A possible safe way to use this API is in conjunction with napi_create_reference, which can be used to guarantee control over the lifetime of the ArrayBuffer. It's also safe to use the returned data buffer within the same callback as long as there are no calls to other APIs that might trigger a GC.

napi_get_buffer_info#

napi_status napi_get_buffer_info(napi_env env,
                                 napi_value value,
                                 void** data,
                                 size_t* length)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing the node::Buffer being queried.
• [out] data: The underlying data buffer of the node::Buffer. If length is 0, this may be NULL or any other pointer value.
• [out] length: Length in bytes of the underlying data buffer.

Returns napi_ok if the API succeeded.

This API is used to retrieve the underlying data buffer of a node::Buffer and its length.

Warning: Use caution while using this API since the underlying data buffer's lifetime is not guaranteed if it's managed by the VM.

napi_get_prototype#

napi_status napi_get_prototype(napi_env env,
                               napi_value object,
                               napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] object: napi_value representing JavaScript Object whose prototype to return. This returns the equivalent of Object.getPrototypeOf (which is not the same as the function's prototype property).
• [out] result: napi_value representing prototype of the given object.

Returns napi_ok if the API succeeded.

napi_get_typedarray_info#

napi_status napi_get_typedarray_info(napi_env env,
                                     napi_value typedarray,
                                     napi_typedarray_type* type,
                                     size_t* length,
                                     void** data,
                                     napi_value* arraybuffer,
                                     size_t* byte_offset)

• [in] env: The environment that the API is invoked under.
• [in] typedarray: napi_value representing the TypedArray whose properties to query.
• [out] type: Scalar datatype of the elements within the TypedArray.
• [out] length: The number of elements in the TypedArray.
• [out] data: The data buffer underlying the TypedArray adjusted by the byte_offset value so that it points to the first element in the TypedArray. If the length of the array is 0, this may be NULL or any other pointer value.
• [out] arraybuffer: The ArrayBuffer underlying the TypedArray.
• [out] byte_offset: The byte offset within the underlying native array at which the first element of the arrays is located. The value for the data parameter has already been adjusted so that data points to the first element in the array. Therefore, the first byte of the native array would be at data - byte_offset.

Returns napi_ok if the API succeeded.

This API returns various properties of a typed array.

Warning: Use caution while using this API since the underlying data buffer is managed by the VM.

napi_get_dataview_info#

napi_status napi_get_dataview_info(napi_env env,
                                   napi_value dataview,
                                   size_t* byte_length,
                                   void** data,
                                   napi_value* arraybuffer,
                                   size_t* byte_offset)

• [in] env: The environment that the API is invoked under.
• [in] dataview: napi_value representing the DataView whose properties to query.
• [out] byte_length: Number of bytes in the DataView.
• [out] data: The data buffer underlying the DataView. If byte_length is 0, this may be NULL or any other pointer value.
• [out] arraybuffer: ArrayBuffer underlying the DataView.
• [out] byte_offset: The byte offset within the data buffer from which to start projecting the DataView.

Returns napi_ok if the API succeeded.

This API returns various properties of a DataView.

napi_get_date_value#

napi_status napi_get_date_value(napi_env env,
                                napi_value value,
                                double* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing a JavaScript Date.
• [out] result: Time value as a double represented as milliseconds since midnight at the beginning of 01 January, 1970 UTC.

This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.

Returns napi_ok if the API succeeded. If a non-date napi_value is passed in it returns napi_date_expected.

This API returns the C double primitive of time value for the given JavaScript Date.

napi_get_value_bool#

napi_status napi_get_value_bool(napi_env env, napi_value value, bool* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript Boolean.
• [out] result: C boolean primitive equivalent of the given JavaScript Boolean.

Returns napi_ok if the API succeeded. If a non-boolean napi_value is passed in it returns napi_boolean_expected.

This API returns the C boolean primitive equivalent of the given JavaScript Boolean.

napi_get_value_double#

napi_status napi_get_value_double(napi_env env,
                                  napi_value value,
                                  double* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript Number.
• [out] result: C double primitive equivalent of the given JavaScript Number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C double primitive equivalent of the given JavaScript Number.

napi_get_value_bigint_int64#

napi_status napi_get_value_bigint_int64(napi_env env,
                                        napi_value value,
                                        int64_t* result,
                                        bool* lossless);

• [in] env: The environment that the API is invoked under
• [in] value: napi_value representing JavaScript BigInt.
• [out] result: C int64_t primitive equivalent of the given JavaScript BigInt.
• [out] lossless: Indicates whether the BigInt value was converted losslessly.

Returns napi_ok if the API succeeded. If a non-BigInt is passed in it returns napi_bigint_expected.

This API returns the C int64_t primitive equivalent of the given JavaScript BigInt. If needed it will truncate the value, setting lossless to false.

napi_get_value_bigint_uint64#

napi_status napi_get_value_bigint_uint64(napi_env env,
                                        napi_value value,
                                        uint64_t* result,
                                        bool* lossless);

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript BigInt.
• [out] result: C uint64_t primitive equivalent of the given JavaScript BigInt.
• [out] lossless: Indicates whether the BigInt value was converted losslessly.

Returns napi_ok if the API succeeded. If a non-BigInt is passed in it returns napi_bigint_expected.

This API returns the C uint64_t primitive equivalent of the given JavaScript BigInt. If needed it will truncate the value, setting lossless to false.

napi_get_value_bigint_words#

napi_status napi_get_value_bigint_words(napi_env env,
                                        napi_value value,
                                        int* sign_bit,
                                        size_t* word_count,
                                        uint64_t* words);

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript BigInt.
• [out] sign_bit: Integer representing if the JavaScript BigInt is positive or negative.
• [in/out] word_count: Must be initialized to the length of the words array. Upon return, it will be set to the actual number of words that would be needed to store this BigInt.
• [out] words: Pointer to a pre-allocated 64-bit word array.

Returns napi_ok if the API succeeded.

This API converts a single BigInt value into a sign bit, 64-bit little-endian array, and the number of elements in the array. sign_bit and words may be both set to NULL, in order to get only word_count.

napi_get_value_external#

napi_status napi_get_value_external(napi_env env,
                                    napi_value value,
                                    void** result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript external value.
• [out] result: Pointer to the data wrapped by the JavaScript external value.

Returns napi_ok if the API succeeded. If a non-external napi_value is passed in it returns napi_invalid_arg.

This API retrieves the external data pointer that was previously passed to napi_create_external().

napi_get_value_int32#

napi_status napi_get_value_int32(napi_env env,
                                 napi_value value,
                                 int32_t* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript Number.
• [out] result: C int32 primitive equivalent of the given JavaScript Number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in napi_number_expected.

This API returns the C int32 primitive equivalent of the given JavaScript Number.

If the number exceeds the range of the 32 bit integer, then the result is truncated to the equivalent of the bottom 32 bits. This can result in a large positive number becoming a negative number if the value is > 2³¹ - 1.

Non-finite number values (NaN, +Infinity, or -Infinity) set the result to zero.

napi_get_value_int64#

napi_status napi_get_value_int64(napi_env env,
                                 napi_value value,
                                 int64_t* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript Number.
• [out] result: C int64 primitive equivalent of the given JavaScript Number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C int64 primitive equivalent of the given JavaScript Number.

Number values outside the range of Number.MIN_SAFE_INTEGER -(2**53 - 1) - Number.MAX_SAFE_INTEGER (2**53 - 1) will lose precision.

Non-finite number values (NaN, +Infinity, or -Infinity) set the result to zero.

napi_get_value_string_latin1#

napi_status napi_get_value_string_latin1(napi_env env,
                                         napi_value value,
                                         char* buf,
                                         size_t bufsize,
                                         size_t* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript string.
• [in] buf: Buffer to write the ISO-8859-1-encoded string into. If NULL is passed in, the length of the string in bytes and excluding the null terminator is returned in result.
• [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.
• [out] result: Number of bytes copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-String napi_value is passed in it returns napi_string_expected.

This API returns the ISO-8859-1-encoded string corresponding the value passed in.

napi_get_value_string_utf8#

napi_status napi_get_value_string_utf8(napi_env env,
                                       napi_value value,
                                       char* buf,
                                       size_t bufsize,
                                       size_t* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript string.
• [in] buf: Buffer to write the UTF8-encoded string into. If NULL is passed in, the length of the string in bytes and excluding the null terminator is returned in result.
• [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.
• [out] result: Number of bytes copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-String napi_value is passed in it returns napi_string_expected.

This API returns the UTF8-encoded string corresponding the value passed in.

napi_get_value_string_utf16#

napi_status napi_get_value_string_utf16(napi_env env,
                                        napi_value value,
                                        char16_t* buf,
                                        size_t bufsize,
                                        size_t* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript string.
• [in] buf: Buffer to write the UTF16-LE-encoded string into. If NULL is passed in, the length of the string in 2-byte code units and excluding the null terminator is returned.
• [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string is truncated and null-terminated.
• [out] result: Number of 2-byte code units copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-String napi_value is passed in it returns napi_string_expected.

This API returns the UTF16-encoded string corresponding the value passed in.

napi_get_value_uint32#

napi_status napi_get_value_uint32(napi_env env,
                                  napi_value value,
                                  uint32_t* result)

• [in] env: The environment that the API is invoked under.
• [in] value: napi_value representing JavaScript Number.
• [out] result: C primitive equivalent of the given napi_value as a uint32_t.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C primitive equivalent of the given napi_value as a uint32_t.

Functions to get global instances#

napi_get_boolean#

napi_status napi_get_boolean(napi_env env, bool value, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The value of the boolean to retrieve.
• [out] result: napi_value representing JavaScript Boolean singleton to retrieve.

Returns napi_ok if the API succeeded.

This API is used to return the JavaScript singleton object that is used to represent the given boolean value.

napi_get_global#

napi_status napi_get_global(napi_env env, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [out] result: napi_value representing JavaScript global object.

Returns napi_ok if the API succeeded.

This API returns the global object.

napi_get_null#

napi_status napi_get_null(napi_env env, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [out] result: napi_value representing JavaScript null object.

Returns napi_ok if the API succeeded.

This API returns the null object.

napi_get_undefined#

napi_status napi_get_undefined(napi_env env, napi_value* result)

• [in] env: The environment that the API is invoked under.
• [out] result: napi_value representing JavaScript Undefined value.

Returns napi_ok if the API succeeded.

This API returns the Undefined object.

Working with JavaScript values and abstract operations#

Node-API exposes a set of APIs to perform some abstract operations on JavaScript values. Some of these operations are documented under Section 7 of the ECMAScript Language Specification.

These APIs support doing one of the following:

1. Coerce JavaScript values to specific JavaScript types (such as Number or String).
2. Check the type of a JavaScript value.
3. Check for equality between two JavaScript values.

napi_coerce_to_bool#

napi_status napi_coerce_to_bool(napi_env env,
                                napi_value value,
                                napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to coerce.
• [out] result: napi_value representing the coerced JavaScript Boolean.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToBoolean() as defined in Section 7.1.2 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_number#

napi_status napi_coerce_to_number(napi_env env,
                                  napi_value value,
                                  napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to coerce.
• [out] result: napi_value representing the coerced JavaScript Number.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToNumber() as defined in Section 7.1.3 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_object#

napi_status napi_coerce_to_object(napi_env env,
                                  napi_value value,
                                  napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to coerce.
• [out] result: napi_value representing the coerced JavaScript Object.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToObject() as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_string#

napi_status napi_coerce_to_string(napi_env env,
                                  napi_value value,
                                  napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to coerce.
• [out] result: napi_value representing the coerced JavaScript String.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToString() as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.

napi_typeof#

napi_status napi_typeof(napi_env env, napi_value value, napi_valuetype* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value whose type to query.
• [out] result: The type of the JavaScript value.

Returns napi_ok if the API succeeded.

• napi_invalid_arg if the type of value is not a known ECMAScript type and value is not an External value.

This API represents behavior similar to invoking the typeof Operator on the object as defined in Section 12.5.5 of the ECMAScript Language Specification. However, there are some differences:

1. It has support for detecting an External value.
2. It detects null as a separate type, while ECMAScript typeof would detect object.

If value has a type that is invalid, an error is returned.

napi_instanceof#

napi_status napi_instanceof(napi_env env,
                            napi_value object,
                            napi_value constructor,
                            bool* result)

• [in] env: The environment that the API is invoked under.
• [in] object: The JavaScript value to check.
• [in] constructor: The JavaScript function object of the constructor function to check against.
• [out] result: Boolean that is set to true if object instanceof constructor is true.

Returns napi_ok if the API succeeded.

This API represents invoking the instanceof Operator on the object as defined in Section 12.10.4 of the ECMAScript Language Specification.

napi_is_array#

napi_status napi_is_array(napi_env env, napi_value value, bool* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to check.
• [out] result: Whether the given object is an array.

Returns napi_ok if the API succeeded.

This API represents invoking the IsArray operation on the object as defined in Section 7.2.2 of the ECMAScript Language Specification.

napi_is_arraybuffer#

napi_status napi_is_arraybuffer(napi_env env, napi_value value, bool* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to check.
• [out] result: Whether the given object is an ArrayBuffer.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is an array buffer.

napi_is_buffer#

napi_status napi_is_buffer(napi_env env, napi_value value, bool* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to check.
• [out] result: Whether the given napi_value represents a node::Buffer object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a buffer.

napi_is_date#

napi_status napi_is_date(napi_env env, napi_value value, bool* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to check.
• [out] result: Whether the given napi_value represents a JavaScript Date object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a date.

napi_is_error#

napi_status napi_is_error(napi_env env, napi_value value, bool* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to check.
• [out] result: Whether the given napi_value represents an Error object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is an Error.

napi_is_typedarray#

napi_status napi_is_typedarray(napi_env env, napi_value value, bool* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to check.
• [out] result: Whether the given napi_value represents a TypedArray.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a typed array.

napi_is_dataview#

napi_status napi_is_dataview(napi_env env, napi_value value, bool* result)

• [in] env: The environment that the API is invoked under.
• [in] value: The JavaScript value to check.
• [out] result: Whether the given napi_value represents a DataView.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a DataView.

napi_strict_equals#

napi_status napi_strict_equals(napi_env env,
                               napi_value lhs,
                               napi_value rhs,
                               bool* result)

• [in] env: The environment that the API is invoked under.
• [in] lhs: The JavaScript value to check.
• [in] rhs: The JavaScript value to check against.
• [out] result: Whether the two napi_value objects are equal.

Returns napi_ok if the API succeeded.

This API represents the invocation of the Strict Equality algorithm as defined in Section 7.2.14 of the ECMAScript Language Specification.

napi_detach_arraybuffer#

napi_status napi_detach_arraybuffer(napi_env env,
                                    napi_value arraybuffer)

• [in] env: The environment that the API is invoked under.
• [in] arraybuffer: The JavaScript ArrayBuffer to be detached.

Returns napi_ok if the API succeeded. If a non-detachable ArrayBuffer is passed in it returns napi_detachable_arraybuffer_expected.

Generally, an ArrayBuffer is non-detachable if it has been detached before. The engine may impose additional conditions on whether an ArrayBuffer is detachable. For example, V8 requires that the ArrayBuffer be external, that is, created with napi_create_external_arraybuffer.

This API represents the invocation of the ArrayBuffer detach operation as defined in Section 24.1.1.3 of the ECMAScript Language Specification.

napi_is_detached_arraybuffer#

napi_status napi_is_detached_arraybuffer(napi_env env,
                                         napi_value arraybuffer,
                                         bool* result)

• [in] env: The environment that the API is invoked under.
• [in] arraybuffer: The JavaScript ArrayBuffer to be checked.
• [out] result: Whether the arraybuffer is detached.

Returns napi_ok if the API succeeded.

The ArrayBuffer is considered detached if its internal data is null.

This API represents the invocation of the ArrayBuffer IsDetachedBuffer operation as defined in Section 24.1.1.2 of the ECMAScript Language Specification.

Working with JavaScript properties#

Node-API exposes a set of APIs to get and set properties on JavaScript objects. Some of these types are documented under Section 7 of the ECMAScript Language Specification.

Properties in JavaScript are represented as a tuple of a key and a value. Fundamentally, all property keys in Node-API can be represented in one of the following forms:

• Named: a simple UTF8-encoded string
• Integer-Indexed: an index value represented by uint32_t
• JavaScript value: these are represented in Node-API by napi_value. This can be a napi_value representing a String, Number, or Symbol.

Node-API values are represented by the type napi_value. Any Node-API call that requires a JavaScript value takes in a napi_value. However, it's the caller's responsibility to make sure that the napi_value in question is of the JavaScript type expected by the API.

The APIs documented in this section provide a simple interface to get and set properties on arbitrary JavaScript objects represented by napi_value.

For instance, consider the following JavaScript code snippet:

const obj = {};
obj.myProp = 123;

The equivalent can be done using Node-API values with the following snippet:

napi_status status = napi_generic_failure;

// const obj = {}
napi_value obj, value;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;

// Create a napi_value for 123
status = napi_create_int32(env, 123, &value);
if (status != napi_ok) return status;

// obj.myProp = 123
status = napi_set_named_property(env, obj, "myProp", value);
if (status != napi_ok) return status;

Indexed properties can be set in a similar manner. Consider the following JavaScript snippet:

const arr = [];
arr[123] = 'hello';

The equivalent can be done using Node-API values with the following snippet:

napi_status status = napi_generic_failure;

// const arr = [];
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;

// Create a napi_value for 'hello'
status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &value);
if (status != napi_ok) return status;

// arr[123] = 'hello';
status = napi_set_element(env, arr, 123, value);
if (status != napi_ok) return status;

Properties can be retrieved using the APIs described in this section. Consider the following JavaScript snippet:

const arr = [];
const value = arr[123];

The following is the approximate equivalent of the Node-API counterpart:

napi_status status = napi_generic_failure;

// const arr = []
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;

// const value = arr[123]
status = napi_get_element(env, arr, 123, &value);
if (status != napi_ok) return status;

Finally, multiple properties can also be defined on an object for performance reasons. Consider the following JavaScript:

const obj = {};
Object.defineProperties(obj, {
  'foo': { value: 123, writable: true, configurable: true, enumerable: true },
  'bar': { value: 456, writable: true, configurable: true, enumerable: true }
});

The following is the approximate equivalent of the Node-API counterpart:

napi_status status = napi_status_generic_failure;

// const obj = {};
napi_value obj;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;

// Create napi_values for 123 and 456
napi_value fooValue, barValue;
status = napi_create_int32(env, 123, &fooValue);
if (status != napi_ok) return status;
status = napi_create_int32(env, 456, &barValue);
if (status != napi_ok) return status;

// Set the properties
napi_property_descriptor descriptors[] = {
  { "foo", NULL, NULL, NULL, NULL, fooValue, napi_writable | napi_configurable, NULL },
  { "bar", NULL, NULL, NULL, NULL, barValue, napi_writable | napi_configurable, NULL }
}
status = napi_define_properties(env,
                                obj,
                                sizeof(descriptors) / sizeof(descriptors[0]),
                                descriptors);
if (status != napi_ok) return status;

Structures#

napi_property_attributes#

typedef enum {
  napi_default = 0,
  napi_writable = 1 << 0,
  napi_enumerable = 1 << 1,
  napi_configurable = 1 << 2,

  // Used with napi_define_class to distinguish static properties
  // from instance properties. Ignored by napi_define_properties.
  napi_static = 1 << 10,

  // Default for class methods.
  napi_default_method = napi_writable | napi_configurable,

  // Default for object properties, like in JS obj[prop].
  napi_default_property = napi_writable |
                          napi_enumerable |
                          napi_configurable,
} napi_property_attributes;

napi_property_attributes are flags used to control the behavior of properties set on a JavaScript object. Other than napi_static they correspond to the attributes listed in Section 6.1.7.1 of the ECMAScript Language Specification. They can be one or more of the following bitflags:

• napi_default: No explicit attributes are set on the property. By default, a property is read only, not enumerable and not configurable.
• napi_writable: The property is writable.
• napi_enumerable: The property is enumerable.
• napi_configurable: The property is configurable as defined in Section 6.1.7.1 of the ECMAScript Language Specification.
• napi_static: The property will be defined as a static property on a class as opposed to an instance property, which is the default. This is used only by napi_define_class. It is ignored by napi_define_properties.
• napi_default_method: Like a method in a JS class, the property is configurable and writeable, but not enumerable.
• napi_default_property: Like a property set via assignment in JavaScript, the property is writable, enumerable, and configurable.

napi_property_descriptor#

typedef struct {
  // One of utf8name or name should be NULL.
  const char* utf8name;
  napi_value name;

  napi_callback method;
  napi_callback getter;
  napi_callback setter;
  napi_value value;

  napi_property_attributes attributes;
  void* data;
} napi_property_descriptor;

• utf8name: Optional String describing the key for the property, encoded as UTF8. One of utf8name or name must be provided for the property.
• name: Optional napi_value that points to a JavaScript string or symbol to be used as the key for the property. One of utf8name or name must be provided for the property.
• value: The value that's retrieved by a get access of the property if the property is a data property. If this is passed in, set getter, setter, method and data to NULL (since these members won't be used).
• getter: A function to call when a get access of the property is performed. If this is passed in, set value and method to NULL (since these members won't be used). The given function is called implicitly by the runtime when the property is accessed from JavaScript code (or if a get on the property is performed using a Node-API call). napi_callback provides more details.
• setter: A function to call when a set access of the property is performed. If this is passed in, set value and method to NULL (since these members won't be used). The given function is called implicitly by the runtime when the property is set from JavaScript code (or if a set on the property is performed using a Node-API call). napi_callback provides more details.
• method: Set this to make the property descriptor object's value property to be a JavaScript function represented by method. If this is passed in, set value, getter and setter to NULL (since these members won't be used). napi_callback provides more details.
• attributes: The attributes associated with the particular property. See napi_property_attributes.
• data: The callback data passed into method, getter and setter if this function is invoked.

Functions#

napi_get_property_names#

napi_status napi_get_property_names(napi_env env,
                                    napi_value object,
                                    napi_value* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object from which to retrieve the properties.
• [out] result: A napi_value representing an array of JavaScript values that represent the property names of the object. The API can be used to iterate over result using napi_get_array_length and napi_get_element.

Returns napi_ok if the API succeeded.

This API returns the names of the enumerable properties of object as an array of strings. The properties of object whose key is a symbol will not be included.

napi_get_all_property_names#

napi_get_all_property_names(napi_env env,
                            napi_value object,
                            napi_key_collection_mode key_mode,
                            napi_key_filter key_filter,
                            napi_key_conversion key_conversion,
                            napi_value* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object from which to retrieve the properties.
• [in] key_mode: Whether to retrieve prototype properties as well.
• [in] key_filter: Which properties to retrieve (enumerable/readable/writable).
• [in] key_conversion: Whether to convert numbered property keys to strings.
• [out] result: A napi_value representing an array of JavaScript values that represent the property names of the object. napi_get_array_length and napi_get_element can be used to iterate over result.

Returns napi_ok if the API succeeded.

This API returns an array containing the names of the available properties of this object.

napi_set_property#

napi_status napi_set_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              napi_value value);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object on which to set the property.
• [in] key: The name of the property to set.
• [in] value: The property value.

Returns napi_ok if the API succeeded.

This API set a property on the Object passed in.

napi_get_property#

napi_status napi_get_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              napi_value* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object from which to retrieve the property.
• [in] key: The name of the property to retrieve.
• [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This API gets the requested property from the Object passed in.

napi_has_property#

napi_status napi_has_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              bool* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object to query.
• [in] key: The name of the property whose existence to check.
• [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in has the named property.

napi_delete_property#

napi_status napi_delete_property(napi_env env,
                                 napi_value object,
                                 napi_value key,
                                 bool* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object to query.
• [in] key: The name of the property to delete.
• [out] result: Whether the property deletion succeeded or not. result can optionally be ignored by passing NULL.

Returns napi_ok if the API succeeded.

This API attempts to delete the key own property from object.

napi_has_own_property#

napi_status napi_has_own_property(napi_env env,
                                  napi_value object,
                                  napi_value key,
                                  bool* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object to query.
• [in] key: The name of the own property whose existence to check.
• [out] result: Whether the own property exists on the object or not.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in has the named own property. key must be a string or a Symbol, or an error will be thrown. Node-API will not perform any conversion between data types.

napi_set_named_property#

napi_status napi_set_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    napi_value value);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object on which to set the property.
• [in] utf8Name: The name of the property to set.
• [in] value: The property value.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_set_property with a napi_value created from the string passed in as utf8Name.

napi_get_named_property#

napi_status napi_get_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    napi_value* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object from which to retrieve the property.
• [in] utf8Name: The name of the property to get.
• [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_get_property with a napi_value created from the string passed in as utf8Name.

napi_has_named_property#

napi_status napi_has_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    bool* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object to query.
• [in] utf8Name: The name of the property whose existence to check.
• [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_has_property with a napi_value created from the string passed in as utf8Name.

napi_set_element#

napi_status napi_set_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             napi_value value);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object from which to set the properties.
• [in] index: The index of the property to set.
• [in] value: The property value.

Returns napi_ok if the API succeeded.

This API sets and element on the Object passed in.

napi_get_element#

napi_status napi_get_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             napi_value* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object from which to retrieve the property.
• [in] index: The index of the property to get.
• [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This API gets the element at the requested index.

napi_has_element#

napi_status napi_has_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             bool* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object to query.
• [in] index: The index of the property whose existence to check.
• [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This API returns if the Object passed in has an element at the requested index.

napi_delete_element#

napi_status napi_delete_element(napi_env env,
                                napi_value object,
                                uint32_t index,
                                bool* result);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object to query.
• [in] index: The index of the property to delete.
• [out] result: Whether the element deletion succeeded or not. result can optionally be ignored by passing NULL.

Returns napi_ok if the API succeeded.

This API attempts to delete the specified index from object.

napi_define_properties#

napi_status napi_define_properties(napi_env env,
                                   napi_value object,
                                   size_t property_count,
                                   const napi_property_descriptor* properties);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object from which to retrieve the properties.
• [in] property_count: The number of elements in the properties array.
• [in] properties: The array of property descriptors.

Returns napi_ok if the API succeeded.

This method allows the efficient definition of multiple properties on a given object. The properties are defined using property descriptors (see napi_property_descriptor). Given an array of such property descriptors, this API will set the properties on the object one at a time, as defined by DefineOwnProperty() (described in Section 9.1.6 of the ECMA-262 specification).

napi_object_freeze#

napi_status napi_object_freeze(napi_env env,
                               napi_value object);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object to freeze.

Returns napi_ok if the API succeeded.

This method freezes a given object. This prevents new properties from being added to it, existing properties from being removed, prevents changing the enumerability, configurability, or writability of existing properties, and prevents the values of existing properties from being changed. It also prevents the object's prototype from being changed. This is described in Section 19.1.2.6 of the ECMA-262 specification.

napi_object_seal#

napi_status napi_object_seal(napi_env env,
                             napi_value object);

• [in] env: The environment that the Node-API call is invoked under.
• [in] object: The object to seal.

Returns napi_ok if the API succeeded.

This method seals a given object. This prevents new properties from being added to it, as well as marking all existing properties as non-configurable. This is described in Section 19.1.2.20 of the ECMA-262 specification.

Working with JavaScript functions#

Node-API provides a set of APIs that allow JavaScript code to call back into native code. Node-APIs that support calling back into native code take in a callback functions represented by the napi_callback type. When the JavaScript VM calls back to native code, the napi_callback function provided is invoked. The APIs documented in this section allow the callback function to do the following:

• Get information about the context in which the callback was invoked.
• Get the arguments passed into the callback.
• Return a napi_value back from the callback.

Additionally, Node-API provides a set of functions which allow calling JavaScript functions from native code. One can either call a function like a regular JavaScript function call, or as a constructor function.

Any non-NULL data which is passed to this API via the data field of the napi_property_descriptor items can be associated with object and freed whenever object is garbage-collected by passing both object and the data to napi_add_finalizer.

napi_call_function#

NAPI_EXTERN napi_status napi_call_function(napi_env env,
                                           napi_value recv,
                                           napi_value func,
                                           size_t argc,
                                           const napi_value* argv,
                                           napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] recv: The this object passed to the called function.
• [in] func: napi_value representing the JavaScript function to be invoked.
• [in] argc: The count of elements in the argv array.
• [in] argv: Array of napi_values representing JavaScript values passed in as arguments to the function.
• [out] result: napi_value representing the JavaScript object returned.

Returns napi_ok if the API succeeded.

This method allows a JavaScript function object to be called from a native add-on. This is the primary mechanism of calling back from the add-on's native code into JavaScript. For the special case of calling into JavaScript after an async operation, see napi_make_callback.

A sample use case might look as follows. Consider the following JavaScript snippet:

function AddTwo(num) {
  return num + 2;
}

Then, the above function can be invoked from a native add-on using the following code:

// Get the function named "AddTwo" on the global object
napi_value global, add_two, arg;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;

status = napi_get_named_property(env, global, "AddTwo", &add_two);
if (status != napi_ok) return;

// const arg = 1337
status = napi_create_int32(env, 1337, &arg);
if (status != napi_ok) return;

napi_value* argv = &arg;
size_t argc = 1;

// AddTwo(arg);
napi_value return_val;
status = napi_call_function(env, global, add_two, argc, argv, &return_val);
if (status != napi_ok) return;

// Convert the result back to a native type
int32_t result;
status = napi_get_value_int32(env, return_val, &result);
if (status != napi_ok) return;

napi_create_function#

napi_status napi_create_function(napi_env env,
                                 const char* utf8name,
                                 size_t length,
                                 napi_callback cb,
                                 void* data,
                                 napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] utf8Name: The name of the function encoded as UTF8. This is visible within JavaScript as the new function object's name property.
• [in] length: The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
• [in] cb: The native function which should be called when this function object is invoked. napi_callback provides more details.
• [in] data: User-provided data context. This will be passed back into the function when invoked later.
• [out] result: napi_value representing the JavaScript function object for the newly created function.

Returns napi_ok if the API succeeded.

This API allows an add-on author to create a function object in native code. This is the primary mechanism to allow calling into the add-on's native code from JavaScript.

The newly created function is not automatically visible from script after this call. Instead, a property must be explicitly set on any object that is visible to JavaScript, in order for the function to be accessible from script.

In order to expose a function as part of the add-on's module exports, set the newly created function on the exports object. A sample module might look as follows:

napi_value SayHello(napi_env env, napi_callback_info info) {
  printf("Hello\n");
  return NULL;
}

napi_value Init(napi_env env, napi_value exports) {
  napi_status status;

  napi_value fn;
  status = napi_create_function(env, NULL, 0, SayHello, NULL, &fn);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "sayHello", fn);
  if (status != napi_ok) return NULL;

  return exports;
}

NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)

Given the above code, the add-on can be used from JavaScript as follows:

const myaddon = require('./addon');
myaddon.sayHello();

The string passed to require() is the name of the target in binding.gyp responsible for creating the .node file.

Any non-NULL data which is passed to this API via the data parameter can be associated with the resulting JavaScript function (which is returned in the result parameter) and freed whenever the function is garbage-collected by passing both the JavaScript function and the data to napi_add_finalizer.

JavaScript Functions are described in Section 19.2 of the ECMAScript Language Specification.

napi_get_cb_info#

napi_status napi_get_cb_info(napi_env env,
                             napi_callback_info cbinfo,
                             size_t* argc,
                             napi_value* argv,
                             napi_value* thisArg,
                             void** data)

• [in] env: The environment that the API is invoked under.
• [in] cbinfo: The callback info passed into the callback function.
• [in-out] argc: Specifies the length of the provided argv array and receives the actual count of arguments.
• [out] argv: Buffer to which the napi_value representing the arguments are copied. If there are more arguments than the provided count, only the requested number of arguments are copied. If there are fewer arguments provided than claimed, the rest of argv is filled with napi_value values that represent undefined.
• [out] this: Receives the JavaScript this argument for the call.
• [out] data: Receives the data pointer for the callback.

Returns napi_ok if the API succeeded.

This method is used within a callback function to retrieve details about the call like the arguments and the this pointer from a given callback info.

napi_get_new_target#

napi_status napi_get_new_target(napi_env env,
                                napi_callback_info cbinfo,
                                napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] cbinfo: The callback info passed into the callback function.
• [out] result: The new.target of the constructor call.

Returns napi_ok if the API succeeded.

This API returns the new.target of the constructor call. If the current callback is not a constructor call, the result is NULL.

napi_new_instance#

napi_status napi_new_instance(napi_env env,
                              napi_value cons,
                              size_t argc,
                              napi_value* argv,
                              napi_value* result)

• [in] env: The environment that the API is invoked under.
• [in] cons: napi_value representing the JavaScript function to be invoked as a constructor.
• [in] argc: The count of elements in the argv array.
• [in] argv: Array of JavaScript values as napi_value representing the arguments to the constructor.
• [out] result: napi_value representing the JavaScript object returned, which in this case is the constructed object.

This method is used to instantiate a new JavaScript value using a given napi_value that represents the constructor for the object. For example, consider the following snippet:

function MyObject(param) {
  this.param = param;
}

const arg = 'hello';
const value = new MyObject(arg);

The following can be approximated in Node-API using the following snippet:

// Get the constructor function MyObject
napi_value global, constructor, arg, value;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;

status = napi_get_named_property(env, global, "MyObject", &constructor);
if (status != napi_ok) return;

// const arg = "hello"
status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &arg);
if (status != napi_ok) return;

napi_value* argv = &arg;
size_t argc = 1;

// const value = new MyObject(arg)
status = napi_new_instance(env, constructor, argc, argv, &value);

Returns napi_ok if the API succeeded.

Object wrap#

Node-API offers a way to "wrap" C++ classes and instances so that the class constructor and methods can be called from JavaScript.

1. The napi_define_class API defines a JavaScript class with constructor, static properties and methods, and instance properties and methods that correspond to the C++ class.
2. When JavaScript code invokes the constructor, the constructor callback uses napi_wrap to wrap a new C++ instance in a JavaScript object, then returns the wrapper object.
3. When JavaScript code invokes a method or property accessor on the class, the corresponding napi_callback C++ function is invoked. For an instance callback, napi_unwrap obtains the C++ instance that is the target of the call.

For wrapped objects it may be difficult to distinguish between a function called on a class prototype and a function called on an instance of a class. A common pattern used to address this problem is to save a persistent reference to the class constructor for later instanceof checks.

napi_value MyClass_constructor = NULL;
status = napi_get_reference_value(env, MyClass::es_constructor, &MyClass_constructor);
assert(napi_ok == status);
bool is_instance = false;
status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance);
assert(napi_ok == status);
if (is_instance) {
  // napi_unwrap() ...
} else {
  // otherwise...
}

The reference must be freed once it is no longer needed.

There are occasions where napi_instanceof() is insufficient for ensuring that a JavaScript object is a wrapper for a certain native type. This is the case especially when wrapped JavaScript objects are passed back into the addon via static methods rather than as the this value of prototype methods. In such cases there is a chance that they may be unwrapped incorrectly.

const myAddon = require('./build/Release/my_addon.node');

// `openDatabase()` returns a JavaScript object that wraps a native database
// handle.
const dbHandle = myAddon.openDatabase();

// `query()` returns a JavaScript object that wraps a native query handle.
const queryHandle = myAddon.query(dbHandle, 'Gimme ALL the things!');

// There is an accidental error in the line below. The first parameter to
// `myAddon.queryHasRecords()` should be the database handle (`dbHandle`), not
// the query handle (`query`), so the correct condition for the while-loop
// should be
//
// myAddon.queryHasRecords(dbHandle, queryHandle)
//
while (myAddon.queryHasRecords(queryHandle, dbHandle)) {
  // retrieve records
}

In the above example myAddon.queryHasRecords() is a method that accepts two arguments. The first is a database handle and the second is a query handle. Internally, it unwraps the first argument and casts the resulting pointer to a native database handle. It then unwraps the second argument and casts the resulting pointer to a query handle. If the arguments are passed in the wrong order, the casts will work, however, there is a good chance that the underlying database operation will fail, or will even cause an invalid memory access.

To ensure that the pointer retrieved from the first argument is indeed a pointer to a database handle and, similarly, that the pointer retrieved from the second argument is indeed a pointer to a query handle, the implementation of queryHasRecords() has to perform a type validation. Retaining the JavaScript class constructor from which the database handle was instantiated and the constructor from which the query handle was instantiated in napi_refs can help, because napi_instanceof() can then be used to ensure that the instances passed into queryHashRecords() are indeed of the correct type.

Unfortunately, napi_instanceof() does not protect against prototype manipulation. For example, the prototype of the database handle instance can be set to the prototype of the constructor for query handle instances. In this case, the database handle instance can appear as a query handle instance, and it will pass the napi_instanceof() test for a query handle instance, while still containing a pointer to a database handle.

To this end, Node-API provides type-tagging capabilities.

A type tag is a 128-bit integer unique to the addon. Node-API provides the napi_type_tag structure for storing a type tag. When such a value is passed along with a JavaScript object stored in a napi_value to napi_type_tag_object(), the JavaScript object will be "marked" with the type tag. The "mark" is invisible on the JavaScript side. When a JavaScript object arrives into a native binding, napi_check_object_type_tag() can be used along with the original type tag to determine whether the JavaScript object was previously "marked" with the type tag. This creates a type-checking capability of a higher fidelity than napi_instanceof() can provide, because such type- tagging survives prototype manipulation and addon unloading/reloading.

Continuing the above example, the following skeleton addon implementation illustrates the use of napi_type_tag_object() and napi_check_object_type_tag().

// This value is the type tag for a database handle. The command
//
//   uuidgen | sed -r -e 's/-//g' -e 's/(.{16})(.*)/0x\1, 0x\2/'
//
// can be used to obtain the two values with which to initialize the structure.
static const napi_type_tag DatabaseHandleTypeTag = {
  0x1edf75a38336451d, 0xa5ed9ce2e4c00c38
};

// This value is the type tag for a query handle.
static const napi_type_tag QueryHandleTypeTag = {
  0x9c73317f9fad44a3, 0x93c3920bf3b0ad6a
};

static napi_value
openDatabase(napi_env env, napi_callback_info info) {
  napi_status status;
  napi_value result;

  // Perform the underlying action which results in a database handle.
  DatabaseHandle* dbHandle = open_database();

  // Create a new, empty JS object.
  status = napi_create_object(env, &result);
  if (status != napi_ok) return NULL;

  // Tag the object to indicate that it holds a pointer to a `DatabaseHandle`.
  status = napi_type_tag_object(env, result, &DatabaseHandleTypeTag);
  if (status != napi_ok) return NULL;

  // Store the pointer to the `DatabaseHandle` structure inside the JS object.
  status = napi_wrap(env, result, dbHandle, NULL, NULL, NULL);
  if (status != napi_ok) return NULL;

  return result;
}

// Later when we receive a JavaScript object purporting to be a database handle
// we can use `napi_check_object_type_tag()` to ensure that it is indeed such a
// handle.

static napi_value
query(napi_env env, napi_callback_info info) {
  napi_status status;
  size_t argc = 2;
  napi_value argv[2];
  bool is_db_handle;

  status = napi_get_cb_info(env, info, &argc, argv, NULL, NULL);
  if (status != napi_ok) return NULL;

  // Check that the object passed as the first parameter has the previously
  // applied tag.
  status = napi_check_object_type_tag(env,
                                      argv[0],
                                      &DatabaseHandleTypeTag,
                                      &is_db_handle);
  if (status != napi_ok) return NULL;

  // Throw a `TypeError` if it doesn't.
  if (!is_db_handle) {
    // Throw a TypeError.
    return NULL;
  }
}

napi_define_class#

napi_status napi_define_class(napi_env env,
                              const char* utf8name,
                              size_t length,
                              napi_callback constructor,
                              void* data,
                              size_t property_count,
                              const napi_property_descriptor* properties,
                              napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] utf8name: Name of the JavaScript constructor function; When wrapping a C++ class, we recommend for clarity that this name be the same as that of the C++ class.
• [in] length: The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
• [in] constructor: Callback function that handles constructing instances of the class. When wrapping a C++ class, this method must be a static member with the napi_callback signature. A C++ class constructor cannot be used. napi_callback provides more details.
• [in] data: Optional data to be passed to the constructor callback as the data property of the callback info.
• [in] property_count: Number of items in the properties array argument.
• [in] properties: Array of property descriptors describing static and instance data properties, accessors, and methods on the class See napi_property_descriptor.
• [out] result: A napi_value representing the constructor function for the class.

Returns napi_ok if the API succeeded.

Defines a JavaScript class, including:

• A JavaScript constructor function that has the class name. When wrapping a corresponding C++ class, the callback passed via constructor can be used to instantiate a new C++ class instance, which can then be placed inside the JavaScript object instance being constructed using napi_wrap.
• Properties on the constructor function whose implementation can call corresponding static data properties, accessors, and methods of the C++ class (defined by property descriptors with the napi_static attribute).
• Properties on the constructor function's prototype object. When wrapping a C++ class, non-static data properties, accessors, and methods of the C++ class can be called from the static functions given in the property descriptors without the napi_static attribute after retrieving the C++ class instance placed inside the JavaScript object instance by using napi_unwrap.

When wrapping a C++ class, the C++ constructor callback passed via constructor should be a static method on the class that calls the actual class constructor, then wraps the new C++ instance in a JavaScript object, and returns the wrapper object. See napi_wrap for details.

The JavaScript constructor function returned from napi_define_class is often saved and used later to construct new instances of the class from native code, and/or to check whether provided values are instances of the class. In that case, to prevent the function value from being garbage-collected, a strong persistent reference to it can be created using napi_create_reference, ensuring that the reference count is kept >= 1.

Any non-NULL data which is passed to this API via the data parameter or via the data field of the napi_property_descriptor array items can be associated with the resulting JavaScript constructor (which is returned in the result parameter) and freed whenever the class is garbage-collected by passing both the JavaScript function and the data to napi_add_finalizer.

napi_wrap#

napi_status napi_wrap(napi_env env,
                      napi_value js_object,
                      void* native_object,
                      napi_finalize finalize_cb,
                      void* finalize_hint,
                      napi_ref* result);

• [in] env: The environment that the API is invoked under.
• [in] js_object: The JavaScript object that will be the wrapper for the native object.
• [in] native_object: The native instance that will be wrapped in the JavaScript object.
• [in] finalize_cb: Optional native callback that can be used to free the native instance when the JavaScript object is ready for garbage-collection. napi_finalize provides more details.
• [in] finalize_hint: Optional contextual hint that is passed to the finalize callback.
• [out] result: Optional reference to the wrapped object.

Returns napi_ok if the API succeeded.

Wraps a native instance in a JavaScript object. The native instance can be retrieved later using napi_unwrap().

When JavaScript code invokes a constructor for a class that was defined using napi_define_class(), the napi_callback for the constructor is invoked. After constructing an instance of the native class, the callback must then call napi_wrap() to wrap the newly constructed instance in the already-created JavaScript object that is the this argument to the constructor callback. (That this object was created from the constructor function's prototype, so it already has definitions of all the instance properties and methods.)

Typically when wrapping a class instance, a finalize callback should be provided that simply deletes the native instance that is received as the data argument to the finalize callback.

The optional returned reference is initially a weak reference, meaning it has a reference count of 0. Typically this reference count would be incremented temporarily during async operations that require the instance to remain valid.

Caution: The optional returned reference (if obtained) should be deleted via napi_delete_reference ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference.

Calling napi_wrap() a second time on an object will return an error. To associate another native instance with the object, use napi_remove_wrap() first.

napi_unwrap#

napi_status napi_unwrap(napi_env env,
                        napi_value js_object,
                        void** result);

• [in] env: The environment that the API is invoked under.
• [in] js_object: The object associated with the native instance.
• [out] result: Pointer to the wrapped native instance.

Returns napi_ok if the API succeeded.

Retrieves a native instance that was previously wrapped in a JavaScript object using napi_wrap().

When JavaScript code invokes a method or property accessor on the class, the corresponding napi_callback is invoked. If the callback is for an instance method or accessor, then the this argument to the callback is the wrapper object; the wrapped C++ instance that is the target of the call can be obtained then by calling napi_unwrap() on the wrapper object.

napi_remove_wrap#

napi_status napi_remove_wrap(napi_env env,
                             napi_value js_object,
                             void** result);

• [in] env: The environment that the API is invoked under.
• [in] js_object: The object associated with the native instance.
• [out] result: Pointer to the wrapped native instance.

Returns napi_ok if the API succeeded.

Retrieves a native instance that was previously wrapped in the JavaScript object js_object using napi_wrap() and removes the wrapping. If a finalize callback was associated with the wrapping, it will no longer be called when the JavaScript object becomes garbage-collected.

napi_type_tag_object#

napi_status napi_type_tag_object(napi_env env,
                                 napi_value js_object,
                                 const napi_type_tag* type_tag);

• [in] env: The environment that the API is invoked under.
• [in] js_object: The JavaScript object to be marked.
• [in] type_tag: The tag with which the object is to be marked.

Returns napi_ok if the API succeeded.

Associates the value of the type_tag pointer with the JavaScript object. napi_check_object_type_tag() can then be used to compare the tag that was attached to the object with one owned by the addon to ensure that the object has the right type.

If the object already has an associated type tag, this API will return napi_invalid_arg.

napi_check_object_type_tag#

napi_status napi_check_object_type_tag(napi_env env,
                                       napi_value js_object,
                                       const napi_type_tag* type_tag,
                                       bool* result);

• [in] env: The environment that the API is invoked under.
• [in] js_object: The JavaScript object whose type tag to examine.
• [in] type_tag: The tag with which to compare any tag found on the object.
• [out] result: Whether the type tag given matched the type tag on the object. false is also returned if no type tag was found on the object.

Returns napi_ok if the API succeeded.

Compares the pointer given as type_tag with any that can be found on js_object. If no tag is found on js_object or, if a tag is found but it does not match type_tag, then result is set to false. If a tag is found and it matches type_tag, then result is set to true.

napi_add_finalizer#

napi_status napi_add_finalizer(napi_env env,
                               napi_value js_object,
                               void* native_object,
                               napi_finalize finalize_cb,
                               void* finalize_hint,
                               napi_ref* result);

• [in] env: The environment that the API is invoked under.
• [in] js_object: The JavaScript object to which the native data will be attached.
• [in] native_object: The native data that will be attached to the JavaScript object.
• [in] finalize_cb: Native callback that will be used to free the native data when the JavaScript object is ready for garbage-collection. napi_finalize provides more details.
• [in] finalize_hint: Optional contextual hint that is passed to the finalize callback.
• [out] result: Optional reference to the JavaScript object.

Returns napi_ok if the API succeeded.

Adds a napi_finalize callback which will be called when the JavaScript object in js_object is ready for garbage collection. This API is similar to napi_wrap() except that:

• the native data cannot be retrieved later using napi_unwrap(),
• nor can it be removed later using napi_remove_wrap(), and
• the API can be called multiple times with different data items in order to attach each of them to the JavaScript object, and
• the object manipulated by the API can be used with napi_wrap().

Caution: The optional returned reference (if obtained) should be deleted via napi_delete_reference ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference.

Simple asynchronous operations#

Addon modules often need to leverage async helpers from libuv as part of their implementation. This allows them to schedule work to be executed asynchronously so that their methods can return in advance of the work being completed. This allows them to avoid blocking overall execution of the Node.js application.

Node-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases.

Node-API defines the napi_async_work structure which is used to manage asynchronous workers. Instances are created/deleted with napi_create_async_work and napi_delete_async_work.

The execute and complete callbacks are functions that will be invoked when the executor is ready to execute and when it completes its task respectively.

The execute function should avoid making any Node-API calls that could result in the execution of JavaScript or interaction with JavaScript objects. Most often, any code that needs to make Node-API calls should be made in complete callback instead. Avoid using the napi_env parameter in the execute callback as it will likely execute JavaScript.

These functions implement the following interfaces:

typedef void (*napi_async_execute_callback)(napi_env env,
                                            void* data);
typedef void (*napi_async_complete_callback)(napi_env env,
                                             napi_status status,
                                             void* data);

When these methods are invoked, the data parameter passed will be the addon-provided void* data that was passed into the napi_create_async_work call.

Once created the async worker can be queued for execution using the napi_queue_async_work function:

napi_status napi_queue_async_work(napi_env env,
                                  napi_async_work work);

napi_cancel_async_work can be used if the work needs to be cancelled before the work has started execution.

After calling napi_cancel_async_work, the complete callback will be invoked with a status value of napi_cancelled. The work should not be deleted before the complete callback invocation, even when it was cancelled.

napi_create_async_work#

napi_status napi_create_async_work(napi_env env,
                                   napi_value async_resource,
                                   napi_value async_resource_name,
                                   napi_async_execute_callback execute,
                                   napi_async_complete_callback complete,
                                   void* data,
                                   napi_async_work* result);

• [in] env: The environment that the API is invoked under.
• [in] async_resource: An optional object associated with the async work that will be passed to possible async_hooks init hooks.
• [in] async_resource_name: Identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
• [in] execute: The native function which should be called to execute the logic asynchronously. The given function is called from a worker pool thread and can execute in parallel with the main event loop thread.
• [in] complete: The native function which will be called when the asynchronous logic is completed or is cancelled. The given function is called from the main event loop thread. napi_async_complete_callback provides more details.
• [in] data: User-provided data context. This will be passed back into the execute and complete functions.
• [out] result: napi_async_work* which is the handle to the newly created async work.

Returns napi_ok if the API succeeded.

This API allocates a work object that is used to execute logic asynchronously. It should be freed using napi_delete_async_work once the work is no longer required.

async_resource_name should be a null-terminated, UTF-8-encoded string.

The async_resource_name identifier is provided by the user and should be representative of the type of async work being performed. It is also recommended to apply namespacing to the identifier, e.g. by including the module name. See the async_hooks documentation for more information.

napi_delete_async_work#

napi_status napi_delete_async_work(napi_env env,
                                   napi_async_work work);

• [in] env: The environment that the API is invoked under.
• [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API frees a previously allocated work object.

This API can be called even if there is a pending JavaScript exception.

napi_queue_async_work#

napi_status napi_queue_async_work(napi_env env,
                                  napi_async_work work);

• [in] env: The environment that the API is invoked under.
• [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API requests that the previously allocated work be scheduled for execution. Once it returns successfully, this API must not be called again with the same napi_async_work item or the result will be undefined.

napi_cancel_async_work#

napi_status napi_cancel_async_work(napi_env env,
                                   napi_async_work work);

• [in] env: The environment that the API is invoked under.
• [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API cancels queued work if it has not yet been started. If it has already started executing, it cannot be cancelled and napi_generic_failure will be returned. If successful, the complete callback will be invoked with a status value of napi_cancelled. The work should not be deleted before the complete callback invocation, even if it has been successfully cancelled.

This API can be called even if there is a pending JavaScript exception.

Custom asynchronous operations#

The simple asynchronous work APIs above may not be appropriate for every scenario. When using any other asynchronous mechanism, the following APIs are necessary to ensure an asynchronous operation is properly tracked by the runtime.

napi_async_init#

napi_status napi_async_init(napi_env env,
                            napi_value async_resource,
                            napi_value async_resource_name,
                            napi_async_context* result)

• [in] env: The environment that the API is invoked under.
• [in] async_resource: Object associated with the async work that will be passed to possible async_hooks init hooks and can be accessed by async_hooks.executionAsyncResource().
• [in] async_resource_name: Identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
• [out] result: The initialized async context.

Returns napi_ok if the API succeeded.

The async_resource object needs to be kept alive until napi_async_destroy to keep async_hooks related API acts correctly. In order to retain ABI compatibility with previous versions, napi_async_contexts are not maintaining the strong reference to the async_resource objects to avoid introducing causing memory leaks. However, if the async_resource is garbage collected by JavaScript engine before the napi_async_context was destroyed by napi_async_destroy, calling napi_async_context related APIs like napi_open_callback_scope and napi_make_callback can cause problems like loss of async context when using the AsyncLocalStoage API.

In order to retain ABI compatibility with previous versions, passing NULL for async_resource does not result in an error. However, this is not recommended as this will result poor results with async_hooks init hooks and async_hooks.executionAsyncResource() as the resource is now required by the underlying async_hooks implementation in order to provide the linkage between async callbacks.

napi_async_destroy#

napi_status napi_async_destroy(napi_env env,
                               napi_async_context async_context);

• [in] env: The environment that the API is invoked under.
• [in] async_context: The async context to be destroyed.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_make_callback#

NAPI_EXTERN napi_status napi_make_callback(napi_env env,
                                           napi_async_context async_context,
                                           napi_value recv,
                                           napi_value func,
                                           size_t argc,
                                           const napi_value* argv,
                                           napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] async_context: Context for the async operation that is invoking the callback. This should normally be a value previously obtained from napi_async_init. In order to retain ABI compatibility with previous versions, passing NULL for async_context does not result in an error. However, this results in incorrect operation of async hooks. Potential issues include loss of async context when using the AsyncLocalStorage API.
• [in] recv: The this object passed to the called function.
• [in] func: napi_value representing the JavaScript function to be invoked.
• [in] argc: The count of elements in the argv array.
• [in] argv: Array of JavaScript values as napi_value representing the arguments to the function.
• [out] result: napi_value representing the JavaScript object returned.

Returns napi_ok if the API succeeded.

This method allows a JavaScript function object to be called from a native add-on. This API is similar to napi_call_function. However, it is used to call from native code back into JavaScript after returning from an async operation (when there is no other script on the stack). It is a fairly simple wrapper around node::MakeCallback.

Note it is not necessary to use napi_make_callback from within a napi_async_complete_callback; in that situation the callback's async context has already been set up, so a direct call to napi_call_function is sufficient and appropriate. Use of the napi_make_callback function may be required when implementing custom async behavior that does not use napi_create_async_work.

Any process.nextTicks or Promises scheduled on the microtask queue by JavaScript during the callback are ran before returning back to C/C++.

napi_open_callback_scope#

NAPI_EXTERN napi_status napi_open_callback_scope(napi_env env,
                                                 napi_value resource_object,
                                                 napi_async_context context,
                                                 napi_callback_scope* result)

• [in] env: The environment that the API is invoked under.
• [in] resource_object: An object associated with the async work that will be passed to possible async_hooks init hooks. This parameter has been deprecated and is ignored at runtime. Use the async_resource parameter in napi_async_init instead.
• [in] context: Context for the async operation that is invoking the callback. This should be a value previously obtained from napi_async_init.
• [out] result: The newly created scope.

There are cases (for example, resolving promises) where it is necessary to have the equivalent of the scope associated with a callback in place when making certain Node-API calls. If there is no other script on the stack the napi_open_callback_scope and napi_close_callback_scope functions can be used to open/close the required scope.

napi_close_callback_scope#

NAPI_EXTERN napi_status napi_close_callback_scope(napi_env env,
                                                  napi_callback_scope scope)

• [in] env: The environment that the API is invoked under.
• [in] scope: The scope to be closed.

This API can be called even if there is a pending JavaScript exception.

Version management#

napi_get_node_version#

typedef struct {
  uint32_t major;
  uint32_t minor;
  uint32_t patch;
  const char* release;
} napi_node_version;

napi_status napi_get_node_version(napi_env env,
                                  const napi_node_version** version);

• [in] env: The environment that the API is invoked under.
• [out] version: A pointer to version information for Node.js itself.

Returns napi_ok if the API succeeded.

This function fills the version struct with the major, minor, and patch version of Node.js that is currently running, and the release field with the value of process.release.name.

The returned buffer is statically allocated and does not need to be freed.

napi_get_version#

napi_status napi_get_version(napi_env env,
                             uint32_t* result);

• [in] env: The environment that the API is invoked under.
• [out] result: The highest version of Node-API supported.

Returns napi_ok if the API succeeded.

This API returns the highest Node-API version supported by the Node.js runtime. Node-API is planned to be additive such that newer releases of Node.js may support additional API functions. In order to allow an addon to use a newer function when running with versions of Node.js that support it, while providing fallback behavior when running with Node.js versions that don't support it:

• Call napi_get_version() to determine if the API is available.
• If available, dynamically load a pointer to the function using uv_dlsym().
• Use the dynamically loaded pointer to invoke the function.
• If the function is not available, provide an alternate implementation that does not use the function.

Memory management#

napi_adjust_external_memory#

NAPI_EXTERN napi_status napi_adjust_external_memory(napi_env env,
                                                    int64_t change_in_bytes,
                                                    int64_t* result);

• [in] env: The environment that the API is invoked under.
• [in] change_in_bytes: The change in externally allocated memory that is kept alive by JavaScript objects.
• [out] result: The adjusted value

Returns napi_ok if the API succeeded.

This function gives V8 an indication of the amount of externally allocated memory that is kept alive by JavaScript objects (i.e. a JavaScript object that points to its own memory allocated by a native module). Registering externally allocated memory will trigger global garbage collections more often than it would otherwise.

Promises#

Node-API provides facilities for creating Promise objects as described in Section 25.4 of the ECMA specification. It implements promises as a pair of objects. When a promise is created by napi_create_promise(), a "deferred" object is created and returned alongside the Promise. The deferred object is bound to the created Promise and is the only means to resolve or reject the Promise using napi_resolve_deferred() or napi_reject_deferred(). The deferred object that is created by napi_create_promise() is freed by napi_resolve_deferred() or napi_reject_deferred(). The Promise object may be returned to JavaScript where it can be used in the usual fashion.

For example, to create a promise and pass it to an asynchronous worker:

napi_deferred deferred;
napi_value promise;
napi_status status;

// Create the promise.
status = napi_create_promise(env, &deferred, &promise);
if (status != napi_ok) return NULL;

// Pass the deferred to a function that performs an asynchronous action.
do_something_asynchronous(deferred);

// Return the promise to JS
return promise;

The above function do_something_asynchronous() would perform its asynchronous action and then it would resolve or reject the deferred, thereby concluding the promise and freeing the deferred:

napi_deferred deferred;
napi_value undefined;
napi_status status;

// Create a value with which to conclude the deferred.
status = napi_get_undefined(env, &undefined);
if (status != napi_ok) return NULL;

// Resolve or reject the promise associated with the deferred depending on
// whether the asynchronous action succeeded.
if (asynchronous_action_succeeded) {
  status = napi_resolve_deferred(env, deferred, undefined);
} else {
  status = napi_reject_deferred(env, deferred, undefined);
}
if (status != napi_ok) return NULL;

// At this point the deferred has been freed, so we should assign NULL to it.
deferred = NULL;

napi_create_promise#

napi_status napi_create_promise(napi_env env,
                                napi_deferred* deferred,
                                napi_value* promise);

• [in] env: The environment that the API is invoked under.
• [out] deferred: A newly created deferred object which can later be passed to napi_resolve_deferred() or napi_reject_deferred() to resolve resp. reject the associated promise.
• [out] promise: The JavaScript promise associated with the deferred object.

Returns napi_ok if the API succeeded.

This API creates a deferred object and a JavaScript promise.

napi_resolve_deferred#

napi_status napi_resolve_deferred(napi_env env,
                                  napi_deferred deferred,
                                  napi_value resolution);

• [in] env: The environment that the API is invoked under.
• [in] deferred: The deferred object whose associated promise to resolve.
• [in] resolution: The value with which to resolve the promise.

This API resolves a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to resolve JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using napi_create_promise() and the deferred object returned from that call must have been retained in order to be passed to this API.

The deferred object is freed upon successful completion.

napi_reject_deferred#

napi_status napi_reject_deferred(napi_env env,
                                 napi_deferred deferred,
                                 napi_value rejection);

• [in] env: The environment that the API is invoked under.
• [in] deferred: The deferred object whose associated promise to resolve.
• [in] rejection: The value with which to reject the promise.

This API rejects a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to reject JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using napi_create_promise() and the deferred object returned from that call must have been retained in order to be passed to this API.

The deferred object is freed upon successful completion.

napi_is_promise#

napi_status napi_is_promise(napi_env env,
                            napi_value value,
                            bool* is_promise);

• [in] env: The environment that the API is invoked under.
• [in] value: The value to examine
• [out] is_promise: Flag indicating whether promise is a native promise object (that is, a promise object created by the underlying engine).

Script execution#

Node-API provides an API for executing a string containing JavaScript using the underlying JavaScript engine.

napi_run_script#

NAPI_EXTERN napi_status napi_run_script(napi_env env,
                                        napi_value script,
                                        napi_value* result);

• [in] env: The environment that the API is invoked under.
• [in] script: A JavaScript string containing the script to execute.
• [out] result: The value resulting from having executed the script.

This function executes a string of JavaScript code and returns its result with the following caveats:

• Unlike eval, this function does not allow the script to access the current lexical scope, and therefore also does not allow to access the module scope, meaning that pseudo-globals such as require will not be available.
• The script can access the global scope. Function and var declarations in the script will be added to the global object. Variable declarations made using let and const will be visible globally, but will not be added to the global object.
• The value of this is global within the script.

libuv event loop#

Node-API provides a function for getting the current event loop associated with a specific napi_env.

napi_get_uv_event_loop#

NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
                                               struct uv_loop_s** loop);

• [in] env: The environment that the API is invoked under.
• [out] loop: The current libuv loop instance.

Asynchronous thread-safe function calls#

JavaScript functions can normally only be called from a native addon's main thread. If an addon creates additional threads, then Node-API functions that require a napi_env, napi_value, or napi_ref must not be called from those threads.

When an addon has additional threads and JavaScript functions need to be invoked based on the processing completed by those threads, those threads must communicate with the addon's main thread so that the main thread can invoke the JavaScript function on their behalf. The thread-safe function APIs provide an easy way to do this.

These APIs provide the type napi_threadsafe_function as well as APIs to create, destroy, and call objects of this type. napi_create_threadsafe_function() creates a persistent reference to a napi_value that holds a JavaScript function which can be called from multiple threads. The calls happen asynchronously. This means that values with which the JavaScript callback is to be called will be placed in a queue, and, for each value in the queue, a call will eventually be made to the JavaScript function.

Upon creation of a napi_threadsafe_function a napi_finalize callback can be provided. This callback will be invoked on the main thread when the thread-safe function is about to be destroyed. It receives the context and the finalize data given during construction, and provides an opportunity for cleaning up after the threads e.g. by calling uv_thread_join(). Aside from the main loop thread, no threads should be using the thread-safe function after the finalize callback completes.

The context given during the call to napi_create_threadsafe_function() can be retrieved from any thread with a call to napi_get_threadsafe_function_context().

Calling a thread-safe function#

napi_call_threadsafe_function() can be used for initiating a call into JavaScript. napi_call_threadsafe_function() accepts a parameter which controls whether the API behaves blockingly. If set to napi_tsfn_nonblocking, the API behaves non-blockingly, returning napi_queue_full if the queue was full, preventing data from being successfully added to the queue. If set to napi_tsfn_blocking, the API blocks until space becomes available in the queue. napi_call_threadsafe_function() never blocks if the thread-safe function was created with a maximum queue size of 0.

napi_call_threadsafe_function() should not be called with napi_tsfn_blocking from a JavaScript thread, because, if the queue is full, it may cause the JavaScript thread to deadlock.

The actual call into JavaScript is controlled by the callback given via the call_js_cb parameter. call_js_cb is invoked on the main thread once for each value that was placed into the queue by a successful call to napi_call_threadsafe_function(). If such a callback is not given, a default callback will be used, and the resulting JavaScript call will have no arguments. The call_js_cb callback receives the JavaScript function to call as a napi_value in its parameters, as well as the void* context pointer used when creating the napi_threadsafe_function, and the next data pointer that was created by one of the secondary threads. The callback can then use an API such as napi_call_function() to call into JavaScript.

The callback may also be invoked with env and call_js_cb both set to NULL to indicate that calls into JavaScript are no longer possible, while items remain in the queue that may need to be freed. This normally occurs when the Node.js process exits while there is a thread-safe function still active.

It is not necessary to call into JavaScript via napi_make_callback() because Node-API runs call_js_cb in a context appropriate for callbacks.

Reference counting of thread-safe functions#

Threads can be added to and removed from a napi_threadsafe_function object during its existence. Thus, in addition to specifying an initial number of threads upon creation, napi_acquire_threadsafe_function can be called to indicate that a new thread will start making use of the thread-safe function. Similarly, napi_release_threadsafe_function can be called to indicate that an existing thread will stop making use of the thread-safe function.

napi_threadsafe_function objects are destroyed when every thread which uses the object has called napi_release_threadsafe_function() or has received a return status of napi_closing in response to a call to napi_call_threadsafe_function. The queue is emptied before the napi_threadsafe_function is destroyed. napi_release_threadsafe_function() should be the last API call made in conjunction with a given napi_threadsafe_function, because after the call completes, there is no guarantee that the napi_threadsafe_function is still allocated. For the same reason, do not use a thread-safe function after receiving a return value of napi_closing in response to a call to napi_call_threadsafe_function. Data associated with the napi_threadsafe_function can be freed in its napi_finalize callback which was passed to napi_create_threadsafe_function(). The parameter initial_thread_count of napi_create_threadsafe_function marks the initial number of aquisitions of the thread-safe functions, instead of calling napi_acquire_threadsafe_function multiple times at creation.

Once the number of threads making use of a napi_threadsafe_function reaches zero, no further threads can start making use of it by calling napi_acquire_threadsafe_function(). In fact, all subsequent API calls associated with it, except napi_release_threadsafe_function(), will return an error value of napi_closing.

The thread-safe function can be "aborted" by giving a value of napi_tsfn_abort to napi_release_threadsafe_function(). This will cause all subsequent APIs associated with the thread-safe function except napi_release_threadsafe_function() to return napi_closing even before its reference count reaches zero. In particular, napi_call_threadsafe_function() will return napi_closing, thus informing the threads that it is no longer possible to make asynchronous calls to the thread-safe function. This can be used as a criterion for terminating the thread. Upon receiving a return value of napi_closing from napi_call_threadsafe_function() a thread must not use the thread-safe function anymore because it is no longer guaranteed to be allocated.

Deciding whether to keep the process running#

Similarly to libuv handles, thread-safe functions can be "referenced" and "unreferenced". A "referenced" thread-safe function will cause the event loop on the thread on which it is created to remain alive until the thread-safe function is destroyed. In contrast, an "unreferenced" thread-safe function will not prevent the event loop from exiting. The APIs napi_ref_threadsafe_function and napi_unref_threadsafe_function exist for this purpose.

Neither does napi_unref_threadsafe_function mark the thread-safe functions as able to be destroyed nor does napi_ref_threadsafe_function prevent it from being destroyed.

napi_create_threadsafe_function#

NAPI_EXTERN napi_status
napi_create_threadsafe_function(napi_env env,
                                napi_value func,
                                napi_value async_resource,
                                napi_value async_resource_name,
                                size_t max_queue_size,
                                size_t initial_thread_count,
                                void* thread_finalize_data,
                                napi_finalize thread_finalize_cb,
                                void* context,
                                napi_threadsafe_function_call_js call_js_cb,
                                napi_threadsafe_function* result);

• [in] env: The environment that the API is invoked under.
• [in] func: An optional JavaScript function to call from another thread. It must be provided if NULL is passed to call_js_cb.
• [in] async_resource: An optional object associated with the async work that will be passed to possible async_hooks init hooks.
• [in] async_resource_name: A JavaScript string to provide an identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
• [in] max_queue_size: Maximum size of the queue. 0 for no limit.
• [in] initial_thread_count: The initial number of acquisitions, i.e. the initial number of threads, including the main thread, which will be making use of this function.
• [in] thread_finalize_data: Optional data to be passed to thread_finalize_cb.
• [in] thread_finalize_cb: Optional function to call when the napi_threadsafe_function is being destroyed.
• [in] context: Optional data to attach to the resulting napi_threadsafe_function.
• [in] call_js_cb: Optional callback which calls the JavaScript function in response to a call on a different thread. This callback will be called on the main thread. If not given, the JavaScript function will be called with no parameters and with undefined as its this value. napi_threadsafe_function_call_js provides more details.
• [out] result: The asynchronous thread-safe JavaScript function.

napi_get_threadsafe_function_context#

NAPI_EXTERN napi_status
napi_get_threadsafe_function_context(napi_threadsafe_function func,
                                     void** result);

• [in] func: The thread-safe function for which to retrieve the context.
• [out] result: The location where to store the context.

This API may be called from any thread which makes use of func.

napi_call_threadsafe_function#

NAPI_EXTERN napi_status
napi_call_threadsafe_function(napi_threadsafe_function func,
                              void* data,
                              napi_threadsafe_function_call_mode is_blocking);

• [in] func: The asynchronous thread-safe JavaScript function to invoke.
• [in] data: Data to send into JavaScript via the callback call_js_cb provided during the creation of the thread-safe JavaScript function.
• [in] is_blocking: Flag whose value can be either napi_tsfn_blocking to indicate that the call should block if the queue is full or napi_tsfn_nonblocking to indicate that the call should return immediately with a status of napi_queue_full whenever the queue is full.

This API should not be called with napi_tsfn_blocking from a JavaScript thread, because, if the queue is full, it may cause the JavaScript thread to deadlock.

This API will return napi_closing if napi_release_threadsafe_function() was called with abort set to napi_tsfn_abort from any thread. The value is only added to the queue if the API returns napi_ok.

This API may be called from any thread which makes use of func.

napi_acquire_threadsafe_function#

NAPI_EXTERN napi_status
napi_acquire_threadsafe_function(napi_threadsafe_function func);

• [in] func: The asynchronous thread-safe JavaScript function to start making use of.

A thread should call this API before passing func to any other thread-safe function APIs to indicate that it will be making use of func. This prevents func from being destroyed when all other threads have stopped making use of it.

This API may be called from any thread which will start making use of func.

napi_release_threadsafe_function#

NAPI_EXTERN napi_status
napi_release_threadsafe_function(napi_threadsafe_function func,
                                 napi_threadsafe_function_release_mode mode);

• [in] func: The asynchronous thread-safe JavaScript function whose reference count to decrement.
• [in] mode: Flag whose value can be either napi_tsfn_release to indicate that the current thread will make no further calls to the thread-safe function, or napi_tsfn_abort to indicate that in addition to the current thread, no other thread should make any further calls to the thread-safe function. If set to napi_tsfn_abort, further calls to napi_call_threadsafe_function() will return napi_closing, and no further values will be placed in the queue.

A thread should call this API when it stops making use of func. Passing func to any thread-safe APIs after having called this API has undefined results, as func may have been destroyed.

This API may be called from any thread which will stop making use of func.

napi_ref_threadsafe_function#

NAPI_EXTERN napi_status
napi_ref_threadsafe_function(napi_env env, napi_threadsafe_function func);

• [in] env: The environment that the API is invoked under.
• [in] func: The thread-safe function to reference.

This API is used to indicate that the event loop running on the main thread should not exit until func has been destroyed. Similar to uv_ref it is also idempotent.

Neither does napi_unref_threadsafe_function mark the thread-safe functions as able to be destroyed nor does napi_ref_threadsafe_function prevent it from being destroyed. napi_acquire_threadsafe_function and napi_release_threadsafe_function are available for that purpose.

This API may only be called from the main thread.

napi_unref_threadsafe_function#

NAPI_EXTERN napi_status
napi_unref_threadsafe_function(napi_env env, napi_threadsafe_function func);

• [in] env: The environment that the API is invoked under.
• [in] func: The thread-safe function to unreference.

This API is used to indicate that the event loop running on the main thread may exit before func is destroyed. Similar to uv_unref it is also idempotent.

This API may only be called from the main thread.

Miscellaneous utilities#

node_api_get_module_file_name#

NAPI_EXTERN napi_status
node_api_get_module_file_name(napi_env env, const char** result);

• [in] env: The environment that the API is invoked under.
• [out] result: A URL containing the absolute path of the location from which the add-on was loaded. For a file on the local file system it will start with file://. The string is null-terminated and owned by env and must thus not be modified or freed.

result may be an empty string if the add-on loading process fails to establish the add-on's file name during loading.