Table of Contents

• ECMAScript Modules
  • Enabling
  • Features
    • Supported
    • Unsupported
  • Notable differences between import and require
    • No NODE_PATH
    • No require.extensions
    • No require.cache
    • URL based paths
  • Interop with existing modules

ECMAScript Modules#

Node contains support for ES Modules based upon the the Node EP for ES Modules.

Not all features of the EP are complete and will be landing as both VM support and implementation is ready. Error messages are still being polished.

Enabling#

The --experimental-modules flag can be used to enable features for loading ESM modules.

Once this has been set, files ending with .mjs will be able to be loaded as ES Modules.

node --experimental-modules my-app.mjs

Features#

Supported#

Only the CLI argument for the main entry point to the program can be an entry point into an ESM graph. In the future import() can be used to create entry points into ESM graphs at run time.

Unsupported#

Feature	Reason
require('./foo.mjs')	ES Modules have differing resolution and timing, use language standard import()
import()	pending newer V8 release used in Node.js
import.meta	pending V8 implementation
Loader Hooks	pending Node.js EP creation/consensus

Notable differences between import and require#

No NODE_PATH#

NODE_PATH is not part of resolving import specifiers. Please use symlinks if this behavior is desired.

No require.extensions#

require.extensions is not used by import. The expectation is that loader hooks can provide this workflow in the future.

No require.cache#

require.cache is not used by import. It has a separate cache.

URL based paths#

ESM are resolved and cached based upon URL semantics. This means that files containing special characters such as # and ? need to be escaped.

Modules will be loaded multiple times if the import specifier used to resolve them have a different query or fragment.

import './foo?query=1'; // loads ./foo with query of "?query=1"
import './foo?query=2'; // loads ./foo with query of "?query=2"

For now, only modules using the file: protocol can be loaded.

Interop with existing modules#

All CommonJS, JSON, and C++ modules can be used with import.

Modules loaded this way will only be loaded once, even if their query or fragment string differs between import statements.

When loaded via import these modules will provide a single default export representing the value of module.exports at the time they finished evaluating.

import fs from 'fs';
fs.readFile('./foo.txt', (err, body) => {
  if (err) {
    console.error(err);
  } else {
    console.log(body);
  }
});