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Using ES modules in Node


Node has had experimental support for EcmaScript modules for a while.

Again, this is one thing I've wanted to explore for a while but have never felt the need to dig deeper. After all, it is experimental and it hasn't been approved for production use.

But wit the release of Node 14 we're coming to the point when module support moved to stable.

So let's explore what it takes to run ES Modules in Node on their own and together with current Node modules.

What will Node consider an ES Module? #

There are certain file extensions and conditions that will cause Node to treat files as ES Modules. These conditions/extensions include:

  • Files ending in .mjs
  • Files ending in .js when the nearest parent package.json file contains a top-level field "type": "module"
  • Files ending in .js when Node runs with the --experimental-modules flag (versions of Node before 14.x)
  • Strings passed in as an argument to --eval or --print, or piped to node via STDIN, with the flag --input-type=module

So, in theory, we could use .mjs for all our ES Modules files but we need to be careful as your server needs to be configured to serve .mjs files as Javascript and I'm not certain all servers are configured to do this out of the box.

File extensions #

Two extensions have special meaning. As we discussed earlier in this post, .mjs will always be treated as an ES Module file

import 'commonjs-package/src/index.mjs';
// Loaded as ES module since .mjs is always 
// loaded as ES module. 

The .cjs extension is the opposite. This file will always load as a Common JS module.

import './legacy-file.cjs';
// Loaded as CommonJS since .cjs is always 
// loaded as CommonJS.

Running Node with module support #

As of Node 12.16.3, the latest LTS Node version as of this writing, the command to run Node with experimental Node Module support is:

node --experimental-modules colors.js

Using this flag we'll be able to run the same code both on Node and on the browser.

If you're interested, keep an eye for announcements about modules in Node. This may change in unexpected ways so check the documentation and be ready.

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