How to publish a module written in ES6 to NPM?

TL;DR - Don't, until ~October 2019. The Node.js Modules Team has asked:

Please do not publish any ES module packages intended for use by Node.js until [October 2019]

2019 May update

Since 2015 when this question was asked, JavaScript support for modules has matured significantly, and is hopefully going to be officially stable in October 2019. All other answers are now obsolete or overly complicated. Here is the current situation and best practice.

ES6 support

99% of ES6 (aka 2015) has been supported by Node since version 6. The current version of Node is 12. All evergreen browsers support the vast majority of ES6 features. ECMAScript is now at version 2019, and the versioning scheme now favors using years.

ES Modules (aka ECMAScript modules) in browsers

All evergreen browsers have been supporting import-ing ES6 modules since 2017. Dynamic imports are supported by Chrome (+ forks like Opera and Samsung Internet) and Safari. Firefox support is slated for the next version, 67.

You no longer need Webpack/rollup/Parcel etc. to load modules. They may be still useful for other purposes, but are not required to load your code. You can directly import URLs pointing to ES modules code.

ES modules in Node

ES modules (.mjs files with import/export) have been supported since Node v8.5.0 by calling node with the --experimental-modules flag. Node v12, released in April 2019, rewrote the experimental modules support. The most visible change is that the file extension needs to be specified by default when importing:

// lib.mjs 

export const hello = 'Hello world!';

// index.mjs:

import { hello } from './lib.mjs';
console.log(hello);

Note the mandatory .mjs extensions throughout. Run as:

node --experimental-modules index.mjs

The Node 12 release is also when the Modules Team asked developers to not publish ES module packages intended for use by Node.js until a solution is found for using packages via both require('pkg') and import 'pkg'. You can still publish native ES modules intended for browsers.

Ecosystem support of native ES modules

As of May 2019, ecosystem support for ES Modules is immature. For example, test frameworks like Jest and Ava don't support --experimental-modules. You need to use a transpiler, and must then decide between using the named import (import { symbol }) syntax (which won't work with most npm packages yet), and the default import syntax (import Package from 'package'), which does work, but not when Babel parses it for packages authored in TypeScript (graphql-tools, node-influx, faast etc.) There is however a workaround that works both with --experimental-modules and if Babel transpiles your code so you can test it with Jest/Ava/Mocha etc:

import * as ApolloServerM from 'apollo-server'; const ApolloServer = ApolloServerM.default || ApolloServerM;

Arguably ugly, but this way you can write your own ES modules code with import/export and run it with node --experimental-modules, without transpilers. If you have dependencies that aren't ESM-ready yet, import them as above, and you'll be able to use test frameworks and other tooling via Babel.

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Previous answer to the question - remember, don't do this until Node solves the require/import issue, hopefully around October 2019.

Publishing ES6 modules to npm, with backwards compatibility

To publish an ES module to npmjs.org so that it can be imported directly, without Babel or other transpilers, simply point the main field in your package.json to the .mjs file, but omit the extension:

{
  "name": "mjs-example",
  "main": "index"
}

That's the only change. By omitting the extension, Node will look first for an mjs file if run with --experimental-modules. Otherwise it will fall back to the .js file, so your existing transpilation process to support older Node versions will work as before — just make sure to point Babel to the .mjs file(s).

Here's the source for a native ES module with backwards compatibility for Node < 8.5.0 that I published to NPM. You can use it right now, without Babel or anything else.

Install the module:

npm install local-iso-dt
# or, yarn add local-iso-dt

Create a test file test.mjs:

import { localISOdt } from 'local-iso-dt/index.mjs';
console.log(localISOdt(), 'Starting job...');

Run node (v8.5.0+) with the --experimental-modules flag:

node --experimental-modules test.mjs

TypeScript

If you develop in TypeScript, you can generate ES6 code and use ES6 modules:

tsc index.js --target es6 --modules es2015

Then, you need to rename *.js output to .mjs, a known issue that will hopefully get fixed soon so tsc can output .mjs files directly.

@Jose is right. There's nothing wrong with publishing ES6/ES2015 to NPM but that may cause trouble, specially if the person using your package is using Webpack, for instance, because normally people ignore the node_modules folder while preprocessing with babel for performance reasons.

So, just use gulp, grunt or simply Node.js to build a lib folder that is ES5.

Here's my build-lib.js script, which I keep in ./tools/ (no gulpor grunt here):

var rimraf = require('rimraf-promise');
var colors = require('colors');
var exec = require('child-process-promise').exec;

console.log('building lib'.green);

rimraf('./lib')
    .then(function (error) {
        let babelCli = 'babel --optional es7.objectRestSpread ./src --out-dir ./lib';
        return exec(babelCli).fail(function (error) {
            console.log(colors.red(error))
        });
    }).then(() => console.log('lib built'.green));

Here's a last advice: You need to add a .npmignore to your project. If npm publish doesn't find this file, it will use .gitignore instead, which will cause you trouble because normally your .gitignore file will exclude ./lib and include ./src, which is exactly the opposite of what you want when you are publishing to NPM. The .npmignore file has basically the same syntax of .gitignore (AFAIK).

The pattern I have seen so far is to keep the es6 files in a src directory and build your stuff in npm's prepublish to the lib directory.

You will need an .npmignore file, similar to .gitignore but ignoring src instead of lib.

I like José's answer. I've noticed several modules follow that pattern already. Here's how you can easily implement it with Babel6. I install babel-cli locally so the build doesn't break if I ever change my global babel version.

.npmignore

/src/

.gitignore

/lib/
/node_modules/

Install Babel

npm install --save-dev babel-core babel-cli babel-preset-es2015

package.json

{
  "main": "lib/index.js",
  "scripts": {
    "prepublish": "babel src --out-dir lib"
  },
  "babel": {
    "presets": ["es2015"]
  }
}