Table of Contents

• Worker Threads

  • worker.isMainThread
  • worker.parentPort
  • worker.threadId
  • worker.workerData
  • Class: MessageChannel

  • Class: MessagePort

    • Event: 'close'
    • Event: 'message'
    • port.close()
    • port.postMessage(value[, transferList])
    • port.ref()
    • port.start()
    • port.unref()

  • Class: Worker

    • new Worker(filename[, options])
    • Event: 'error'
    • Event: 'exit'
    • Event: 'message'
    • Event: 'online'
    • worker.postMessage(value[, transferList])
    • worker.ref()
    • worker.stderr
    • worker.stdin
    • worker.stdout
    • worker.terminate([callback])
    • worker.threadId
    • worker.unref()

Worker Threads#

The worker_threads module enables the use of threads that execute JS code in parallel. To access it:

const worker = require('worker_threads');

Workers (threads) are useful for performing CPU-intensive JavaScript operations. They will not help much with I/O-intensive work. Node.js’s built-in asynchronous I/O operations are more efficient than Workers can be.

Unlike child_process or cluster, worker_threads can share memory. They do so by transferring ArrayBuffer instances or sharing SharedArrayBuffer instances.

const {
  Worker, isMainThread, parentPort, workerData
} = require('worker_threads');

if (isMainThread) {
  module.exports = function parseJSAsync(script) {
    return new Promise((resolve, reject) => {
      const worker = new Worker(__filename, {
        workerData: script
      });
      worker.on('message', resolve);
      worker.on('error', reject);
      worker.on('exit', (code) => {
        if (code !== 0)
          reject(new Error(`Worker stopped with exit code ${code}`));
      });
    });
  };
} else {
  const { parse } = require('some-js-parsing-library');
  const script = workerData;
  parentPort.postMessage(parse(script));
}

The above example spawns a Worker thread for each parse() call. In actual practice, use a pool of Workers instead for these kinds of tasks. Otherwise, the overhead of creating Workers would likely exceed their benefit.

worker.isMainThread#

• <boolean>

Is true if this code is not running inside of a Worker thread.

const { Worker, isMainThread } = require('worker_threads');

if (isMainThread) {
  // This re-loads the current file inside a Worker instance.
  new Worker(__filename);
} else {
  console.log('Inside Worker!');
  console.log(isMainThread);  // Prints 'false'.
}

worker.parentPort#

• <null> | <MessagePort>

If this thread was spawned as a Worker, this will be a MessagePort allowing communication with the parent thread. Messages sent using parentPort.postMessage() will be available in the parent thread using worker.on('message'), and messages sent from the parent thread using worker.postMessage() will be available in this thread using parentPort.on('message').

const { Worker, isMainThread, parentPort } = require('worker_threads');

if (isMainThread) {
  const worker = new Worker(__filename);
  worker.once('message', (message) => {
    console.log(message);  // Prints 'Hello, world!'.
  });
  worker.postMessage('Hello, world!');
} else {
  // When a message from the parent thread is received, send it back:
  parentPort.once('message', (message) => {
    parentPort.postMessage(message);
  });
}

worker.threadId#

• <integer>

An integer identifier for the current thread. On the corresponding worker object (if there is any), it is available as worker.threadId. This value is unique for each Worker instance inside a single process.

worker.workerData#

An arbitrary JavaScript value that contains a clone of the data passed to this thread’s Worker constructor.

The data is cloned as if using postMessage(), according to the HTML structured clone algorithm.

const { Worker, isMainThread, workerData } = require('worker_threads');

if (isMainThread) {
  const worker = new Worker(__filename, { workerData: 'Hello, world!' });
} else {
  console.log(workerData);  // Prints 'Hello, world!'.
}

Class: MessageChannel#

Instances of the worker.MessageChannel class represent an asynchronous, two-way communications channel. The MessageChannel has no methods of its own. new MessageChannel() yields an object with port1 and port2 properties, which refer to linked MessagePort instances.

const { MessageChannel } = require('worker_threads');

const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log('received', message));
port2.postMessage({ foo: 'bar' });
// Prints: received { foo: 'bar' } from the `port1.on('message')` listener

Class: MessagePort#

• Extends: <EventEmitter>

Instances of the worker.MessagePort class represent one end of an asynchronous, two-way communications channel. It can be used to transfer structured data, memory regions and other MessagePorts between different Workers.

With the exception of MessagePorts being EventEmitters rather than EventTargets, this implementation matches browser MessagePorts.

Event: 'close'#

The 'close' event is emitted once either side of the channel has been disconnected.

const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();

// Prints:
//   foobar
//   closed!
port2.on('message', (message) => console.log(message));
port2.on('close', () => console.log('closed!'));

port1.postMessage('foobar');
port1.close();

Event: 'message'#

• value <any> The transmitted value

The 'message' event is emitted for any incoming message, containing the cloned input of port.postMessage().

Listeners on this event will receive a clone of the value parameter as passed to postMessage() and no further arguments.

port.close()#

Disables further sending of messages on either side of the connection. This method can be called when no further communication will happen over this MessagePort.

The 'close' event will be emitted on both MessagePort instances that are part of the channel.

port.postMessage(value[, transferList])#

• value <any>
• transferList <Object[]>

Sends a JavaScript value to the receiving side of this channel. value will be transferred in a way which is compatible with the HTML structured clone algorithm.

In particular, the significant differences to JSON are:

• value may contain circular references.
• value may contain instances of builtin JS types such as RegExps, BigInts, Maps, Sets, etc.
• value may contained typed arrays, both using ArrayBuffers and SharedArrayBuffers.
• value may contain WebAssembly.Module instances.
• value may not contain native (C++-backed) objects other than MessagePorts.

const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();

port1.on('message', (message) => console.log(message));

const circularData = {};
circularData.foo = circularData;
// Prints: { foo: [Circular] }
port2.postMessage(circularData);

transferList may be a list of ArrayBuffer and MessagePort objects. After transferring, they will not be usable on the sending side of the channel anymore (even if they are not contained in value). Unlike with child processes, transferring handles such as network sockets is currently not supported.

If value contains SharedArrayBuffer instances, those will be accessible from either thread. They cannot be listed in transferList.

value may still contain ArrayBuffer instances that are not in transferList; in that case, the underlying memory is copied rather than moved.

const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();

port1.on('message', (message) => console.log(message));

const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]);
// This posts a copy of `uint8Array`:
port2.postMessage(uint8Array);
// This does not copy data, but renders `uint8Array` unusable:
port2.postMessage(uint8Array, [ uint8Array.buffer ]);

// The memory for the `sharedUint8Array` will be accessible from both the
// original and the copy received by `.on('message')`:
const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
port2.postMessage(sharedUint8Array);

// This transfers a freshly created message port to the receiver.
// This can be used, for example, to create communication channels between
// multiple `Worker` threads that are children of the same parent thread.
const otherChannel = new MessageChannel();
port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]);

Because the object cloning uses the structured clone algorithm, non-enumerable properties, property accessors, and object prototypes are not preserved. In particular, Buffer objects will be read as plain Uint8Arrays on the receiving side.

The message object will be cloned immediately, and can be modified after posting without having side effects.

For more information on the serialization and deserialization mechanisms behind this API, see the serialization API of the v8 module.

port.ref()#

Opposite of unref(). Calling ref() on a previously unref()ed port will not let the program exit if it's the only active handle left (the default behavior). If the port is ref()ed, calling ref() again will have no effect.

If listeners are attached or removed using .on('message'), the port will be ref()ed and unref()ed automatically depending on whether listeners for the event exist.

port.start()#

Starts receiving messages on this MessagePort. When using this port as an event emitter, this will be called automatically once 'message' listeners are attached.

This method exists for parity with the Web MessagePort API. In Node.js, it is only useful for ignoring messages when no event listener is present.

port.unref()#

Calling unref() on a port will allow the thread to exit if this is the only active handle in the event system. If the port is already unref()ed calling unref() again will have no effect.

If listeners are attached or removed using .on('message'), the port will be ref()ed and unref()ed automatically depending on whether listeners for the event exist.

Class: Worker#

• Extends: <EventEmitter>

The Worker class represents an independent JavaScript execution thread. Most Node.js APIs are available inside of it.

Notable differences inside a Worker environment are:

• The process.stdin, process.stdout and process.stderr may be redirected by the parent thread.
• The require('worker_threads').isMainThread property is set to false.
• The require('worker_threads').parentPort message port is available,
• process.exit() does not stop the whole program, just the single thread, and process.abort() is not available.
• process.chdir() and process methods that set group or user ids are not available.
• process.env is a read-only reference to the environment variables.
• process.title cannot be modified.
• Signals will not be delivered through process.on('...').
• Execution may stop at any point as a result of worker.terminate() being invoked.
• IPC channels from parent processes are not accessible.
• The trace_events module is not supported.
• Native add-ons can only be loaded from multiple threads if they fulfill certain conditions.

Creating Worker instances inside of other Workers is possible.

Like Web Workers and the cluster module, two-way communication can be achieved through inter-thread message passing. Internally, a Worker has a built-in pair of MessagePorts that are already associated with each other when the Worker is created. While the MessagePort object on the parent side is not directly exposed, its functionalities are exposed through worker.postMessage() and the worker.on('message') event on the Worker object for the parent thread.

To create custom messaging channels (which is encouraged over using the default global channel because it facilitates separation of concerns), users can create a MessageChannel object on either thread and pass one of the MessagePorts on that MessageChannel to the other thread through a pre-existing channel, such as the global one.

See port.postMessage() for more information on how messages are passed, and what kind of JavaScript values can be successfully transported through the thread barrier.

const assert = require('assert');
const {
  Worker, MessageChannel, MessagePort, isMainThread, parentPort
} = require('worker_threads');
if (isMainThread) {
  const worker = new Worker(__filename);
  const subChannel = new MessageChannel();
  worker.postMessage({ hereIsYourPort: subChannel.port1 }, [subChannel.port1]);
  subChannel.port2.on('message', (value) => {
    console.log('received:', value);
  });
} else {
  parentPort.once('message', (value) => {
    assert(value.hereIsYourPort instanceof MessagePort);
    value.hereIsYourPort.postMessage('the worker is sending this');
    value.hereIsYourPort.close();
  });
}

new Worker(filename[, options])#

• filename <string> The path to the Worker’s main script. Must be either an absolute path or a relative path (i.e. relative to the current working directory) starting with ./ or ../. If options.eval is true, this is a string containing JavaScript code rather than a path.

• options <Object>

  • eval <boolean> If true, interpret the first argument to the constructor as a script that is executed once the worker is online.
  • workerData <any> Any JavaScript value that will be cloned and made available as require('worker_threads').workerData. The cloning will occur as described in the HTML structured clone algorithm, and an error will be thrown if the object cannot be cloned (e.g. because it contains functions).
  • stdin <boolean> If this is set to true, then worker.stdin will provide a writable stream whose contents will appear as process.stdin inside the Worker. By default, no data is provided.
  • stdout <boolean> If this is set to true, then worker.stdout will not automatically be piped through to process.stdout in the parent.
  • stderr <boolean> If this is set to true, then worker.stderr will not automatically be piped through to process.stderr in the parent.
  • execArgv <string[]> List of node CLI options passed to the worker. V8 options (such as --max-old-space-size) and options that affect the process (such as --title) are not supported. If set, this will be provided as process.execArgv inside the worker. By default, options will be inherited from the parent thread.

Event: 'error'#

• err <Error>

The 'error' event is emitted if the worker thread throws an uncaught exception. In that case, the worker will be terminated.

Event: 'exit'#

• exitCode <integer>

The 'exit' event is emitted once the worker has stopped. If the worker exited by calling process.exit(), the exitCode parameter will be the passed exit code. If the worker was terminated, the exitCode parameter will be 1.

Event: 'message'#

• value <any> The transmitted value

The 'message' event is emitted when the worker thread has invoked require('worker_threads').parentPort.postMessage(). See the port.on('message') event for more details.

Event: 'online'#

The 'online' event is emitted when the worker thread has started executing JavaScript code.

worker.postMessage(value[, transferList])#

• value <any>
• transferList <Object[]>

Send a message to the worker that will be received via require('worker_threads').parentPort.on('message'). See port.postMessage() for more details.

worker.ref()#

Opposite of unref(), calling ref() on a previously unref()ed worker will not let the program exit if it's the only active handle left (the default behavior). If the worker is ref()ed, calling ref() again will have no effect.

worker.stderr#

• <stream.Readable>

This is a readable stream which contains data written to process.stderr inside the worker thread. If stderr: true was not passed to the Worker constructor, then data will be piped to the parent thread's process.stderr stream.

worker.stdin#

• <null> | <stream.Writable>

If stdin: true was passed to the Worker constructor, this is a writable stream. The data written to this stream will be made available in the worker thread as process.stdin.

worker.stdout#

• <stream.Readable>

This is a readable stream which contains data written to process.stdout inside the worker thread. If stdout: true was not passed to the Worker constructor, then data will be piped to the parent thread's process.stdout stream.

worker.terminate([callback])#

• callback <Function>

  • err <Error>
  • exitCode <integer>

Stop all JavaScript execution in the worker thread as soon as possible. callback is an optional function that is invoked once this operation is known to have completed.

Warning: Currently, not all code in the internals of Node.js is prepared to expect termination at arbitrary points in time and may crash if it encounters that condition. Consequently, only call .terminate() if it is known that the Worker thread is not accessing Node.js core modules other than what is exposed in the worker module.

worker.threadId#

• <integer>

An integer identifier for the referenced thread. Inside the worker thread, it is available as require('worker_threads').threadId. This value is unique for each Worker instance inside a single process.

worker.unref()#

Calling unref() on a worker will allow the thread to exit if this is the only active handle in the event system. If the worker is already unref()ed calling unref() again will have no effect.