How to publish a module written in ES6 to NPM?

Question

I was about to publish a module to NPM, when I thought about rewriting it in ES6, to both future-proof it, and learn ES6. I've used Babel to transpile to ES5, and run tests. But I'm not sure how to proceed:

1. Do I transpile, and publish the resulting /out folder to NPM?
2. Do I include the result folder in my Github repo?
3. Or do I maintain 2 repos, one with the ES6 code + gulp script for Github, and one with the transpiled results + tests for NPM?

In short: what steps do I need to take to publish a module written in ES6 to NPM, while still allowing people to browse/fork the original code?

Grandniece

I have been struggling with this decision lately. I am seeing the answer you marked as correct by José being the consensus also. – Aubyn

Here's my 2018 answer, taking into account the progress with module support since 2015. – Pumpkin

I'd love if I could do the opposite. Use an ES Module to import an NPM module, but these are the only results I get. – Bookout

Answer 1

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.

Incontrollable

Do you have example repository? – Mooneyham

@JamesAkwuh Note that you will likely want to change the "start" and "build" commands in the package.json to use the relative path of the babel-cli: ./node_modules/babel-cli/bin/babel.js -s inline -d lib -w src. This should ensure that installs don't fail when deploying to new environments. – Hazzard

@Hazzard npm handles it – Unexacting

“If there's no .npmignore file, but there is a .gitignore file, then npm will ignore the stuff matched by the .gitignore file.” official npm docs – Alloway

Instead of .npmignore you can use the files field in package.json. It lets you specify exactly the files you want to publish, instead of hunting for random files you don't want to publish. – Pumpkin

I have a boilerplate repo that does exactly as what's mentioned in this answer and it's the recommended approach as well - npm-module-boilerplate – Crumpet

Answer 2

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"]
  }
}

Fanaticize

Any commands in the scripts will have node_modules/.bin added to their $PATH and since babel-cli installs a binary to node_modules/.bin/babel there is no need to reference the command by path. – Fessler

Please notice that prepublish is problematic because it could run at install time (github.com/npm/npm/issues/3059), prefer the more idiomatic version script hook (docs.npmjs.com/cli/version) – Sixfold

@mattecapu it seems like the problem with prepublish is still there. At the moment I think manually compile the src directory and npm publish is the way to go. – Pastille

You can use prepublishOnly script hook (see docs.npmjs.com/misc/scripts#prepublish-and-prepare). Note that in version 5 of npm this should function as expected, but for now (assuming you're using npm v4+) this should work. – Raster

@marius-craciunoiu, I still struggle, on which end the es5 transpilation is meant to happen? – I can see loads of projects, building (directly or indirectly through build target) in the prepublish script. But nobody (except for <1% I deem in error) commits the lib/ folder, as I can see from each respective repository... — thus prepublish is more of a syntax check, not a pre-rendering (or: „pre-built binaries“) ? – Alloway

How does package.json - "files" (docs) interfere with .npmignore? – Alloway

@FrankNocke from the docs, "even if they would be picked up by the files array", so "if in files and not in ignore", so you could, eg add lib to files and *.doc.xml to .npmignore. – Coast

@FrankNocke prepublish runs before publish (obv.), which pushes stuff to npm (or wherever you configure). So it's for building what goes in the NPM package, even if it's not checked in. – Coast

A good article (github.com/bookercodes/articles/blob/master/…) suggests putting babel compilation into the "build" script, while executing it from "prepublish": "npm run build" or "yarn build" if your use yarn instead. – Commutative

Note that the built-in prepublish script has been deprecated in favour of prepare. See here for an overview of the change: iamakulov.com/notes/npm-4-prepublish – Urology

Answer 3

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.

---

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.

Pumpkin

Saying "All evergreen browsers support the vast majority of ES6 features." doesn't mean much when you look at the data and realise that es6 support in browsers only reaches about 80% of all users. – Folketing

Currently, the ecosystem is definitely not mature enough for this. The Node.js team with the release of v12 specifically asked: "Please do not publish any ES module packages intended for use by Node.js until this is resolved." 2ality.com/2019/04/nodejs-esm-impl.html#es-modules-on-npm Mocha doesn't natively support .mjs files. Many-many libraries (e.g. create-react-app, react-apollo, graphql-js) had issues with dependencies containing mjs files. Node.js plans to roll out official support in October 2019 which is the earliest I would seriously revisit this. – Gironde

"All other answers are now obsolete or overly complicated. Here is the current situation and best practice." This seems highly subjective and definitely does not reflect the state of npm packages being published in 2021. – Urology

For readers in 2022: This answer probably needs an update. In package.json one can set "type": "module" to read .js as .mjs by default. And maybe try to simplify it a bit, it's very verbose and misleading for me. – Latishalatitude

Answer 4

@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).

Turnover

Instead of .npmignore you can use the files field in package.json. It lets you specify exactly the files you want to publish, instead of hunting for random files you don't want to publish. – Pumpkin

Wont this break tree-shaking? – Weiweibel

I don't recommend using .npmignore, try package.json's files instead, see: github.com/c-hive/guides/blob/master/js/… – Gironde

@thisismydesign: that's exactly what I had recommended in my comment above..? – Pumpkin

My bad, didn't notice :) – Gironde

Answer 5

Following José and Marius's approach, (with update of Babel's latest version in 2019): Keep the latest JavaScript files in a src directory, and build with npm's prepublish script and output to the lib directory.

.npmignore

/src

.gitignore

/lib
/node_modules

Install Babel (version 7.5.5 in my case)

$ npm install @babel/core @babel/cli @babel/preset-env --save-dev

package.json

{
  "name": "latest-js-to-npm",
  "version": "1.0.0",
  "description": "Keep the latest JavaScript files in a src directory and build with npm's prepublish script and output to the lib directory.",
  "main": "lib/index.js",
  "scripts": {
    "prepublish": "babel src -d lib"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@babel/cli": "^7.5.5",
    "@babel/core": "^7.5.5",
    "@babel/preset-env": "^7.5.5"
  },
  "babel": {
    "presets": [
      "@babel/preset-env"
    ]
  }
}

And I have src/index.js which uses the arrow function:

"use strict";

let NewOneWithParameters = (a, b) => {
  console.log(a + b); // 30
};
NewOneWithParameters(10, 20);

Here is the repo on GitHub.

Now you can publish the package:

$ npm publish
...
> [email protected] prepublish .
> babel src -d lib

Successfully compiled 1 file with Babel.
...

Before the package is published to npm, you will see that lib/index.js has been generated, which is transpiled to es5:

"use strict";

var NewOneWithParameters = function NewOneWithParameters(a, b) {
  console.log(a + b); // 30
};

NewOneWithParameters(10, 20);

[Update for Rollup bundler]

As asked by @kyw, how would you integrate Rollup bundler?

First, install rollup and rollup-plugin-babel

npm install -D rollup rollup-plugin-babel

Second, create rollup.config.js in the project root directory

import babel from "rollup-plugin-babel";

export default {
  input: "./src/index.js",
  output: {
    file: "./lib/index.js",
    format: "cjs",
    name: "bundle"
  },
  plugins: [
    babel({
      exclude: "node_modules/**"
    })
  ]
};

Lastly, update prepublish in package.json

{
  ...
  "scripts": {
    "prepublish": "rollup -c"
  },
  ...
}

Now you can run npm publish, and before the package is published to npm, you will see that lib/index.js has been generated, which is transpiled to es5:

'use strict';

var NewOneWithParameters = function NewOneWithParameters(a, b) {
  console.log(a + b); // 30
};

NewOneWithParameters(10, 20);

Note: by the way, you no longer need @babel/cli if you are using the Rollup bundler. You can safely uninstall it:

npm uninstall @babel/cli

Wholism

How would you integrate Rollup bundler? – Hounding

@kyw, regarding how to integrate Rollup bundler, see my updated answer. – Wholism

December 2019 update --> github.com/rollup/rollup/blob/… – Hattie

Thanks for this! Just to clarify, to integrate your function in another project you can just use import NewOneWithParameters from 'latest-js-to-npm'; ? So in your code you don't actually need export? – Bloodhound

Answer 6

If you want to see this in action in a very simple small open source Node module then take a look at nth-day (which I started - also other contributors). Look in the package.json file and at the prepublish step which will lead you to where and how to do this. If you clone that module you can run it locally and use it as a template for yous.

Deliquescence

Answer 7

Node.js 13.2.0+ supports ESM without the experimental flag and there're a few options to publish hybrid (ESM and CommonJS) NPM packages (depending on the level of backward compatibility needed): https://2ality.com/2019/10/hybrid-npm-packages.html

I recommend going the full backward compatibility way to make the usage of your package easier. This could look as follows:

The hybrid package has the following files:

mypkg/
  package.json
  esm/
    entry.js
  commonjs/
    package.json
    entry.js

mypkg/package.json

{
  "type": "module",
  "main": "./commonjs/entry.js",
  "exports": {
    "./esm": "./esm/entry.js"
  },
  "module": "./esm/entry.js",
  ···
}

mypkg/commonjs/package.json

{
  "type": "commonjs"
}

Importing from CommonJS:

const {x} = require('mypkg');

Importing from ESM:

import {x} from 'mypkg/esm';

We did an investigation into ESM support in 05.2019 and found that a lot of libraries were lacking support (hence the recommendation for backward compatibility):

• esm package's support doesn't align with Node's which causes issues
  • "Builtin require cannot sideload .mjs files." https://github.com/standard-things/esm#loading, https://github.com/standard-things/esm/issues/498#issuecomment-403496745
  • "The .mjs file extension should not be the thing developers reach for if they want interop or ease of use. It's available since it's in --experimental-modules but since it's not fully baked I can't commit to any enhancements to it." https://github.com/standard-things/esm/issues/498#issuecomment-403655466
• mocha doesn't have native support for .mjs files
  • Update 2020-01-13: Mocha released experimental support in [email protected]
• Many high-profile projects had issues with .mjs files:
  • create-react-app
  • react-apollo
  • graphql-js
  • inferno

Gironde

I can't seem to import globally installed ES6 modules in the node_modules folder (provided by npm root -g). Are we really not supposed to be able to do that? I am really confused. I know npm link can solve the problem by linking the module to my local node_modules folder, but I want to know why importing global node modules is not supported. – Amazing

Answering myself, I guess it will never be supported: github.com/nodejs/node-eps/blob/master/… It's a really stupid decision though, would be easy to support... – Amazing

@JoakimL.Christiansen Namaste. Having similar hardship due to the completely misarticulated official stance with regard to modules by the node community, I followed that link, and yes, I was disappointed, but not being satisfied, I checked the publish date of that linked article... 9 Aug 2017. Just pointing that out as the linked article could be a red herring so to speak... – Insalivate

Answer 8

The main key in package.json decides the entry point to the package once it's published. So you can put your Babel's output wherever you want and just have to mention the right path in main key.

"main": "./lib/index.js",

Here's a well written article on how to publish an npm package

https://codeburst.io/publish-your-own-npm-package-ff918698d450

Here's a sample repo you can use for reference

https://github.com/flexdinesh/npm-module-boilerplate

Crumpet

That post omits the fact that you can (and should) publish to NPM modules authored in ES6 and importable directly without requiring Babel or any other transpilers. – Pumpkin

Answer 9

The two criteria of an NPM package is that it is usable with nothing more than a require( 'package' ) and does something software-ish.

If you fulfill those two requirements, you can do whatever you wish. Even if the module is written in ES6, if the end user doesn't need to know that, I would transpile it for now to get maximum support.

However, if like koa, your module requires compatibility with users using ES6 features, then perhaps the two package solution would be a better idea.

Takeaway

1. Only publish as much code as you need to make require( 'your-package' ) work.
2. Unless the between ES5 & 6 matters to the user, only publish 1 package. Transpile it if you must.

Excoriation

This doesn't seem to answer the question. I think the OP is trying to figure out how to structure their Github repo and what to publish to NPM and all you've kind of said is that they can do whatever they want. The OP wants specific recommendations on a good practice for this situation. – Khedive

@Khedive I disagree. I've recommended that he transpile, and only publish the files that are required for require( 'package' ) to work. I'll edit my answer to make this more clear. That said, Jose's answer is much better than my own. – Excoriation

José's answer is very good, but I appreciate this one for explicitly outlining good rules-of-thumb for when/why to use one vs. two packages. – Sallee

Answer 10

A few extra notes for anyone, using own modules directly from github, not going through published modules:

The (widely used) "prepublish" hook is not doing anything for you.

Best thing one can do (if plans to rely on github repos, not published stuff):

• unlist src from .npmignore (in other words: allow it). If you don't have an .npmignore, remember: A copy of .gitignore will be used instead in the installed location, as ls node_modules/yourProject will show you.
• make sure, babel-cli is a depenency in your module, not just a devDepenceny since you are indeed building on the consuming machine aka at the App developers computer, who is using your module

• do the build thing, in the install hook i.e.:

  "install": "babel src -d lib -s"

(no added value in trying anything "preinstall", i.e. babel-cli might be missing)

Alloway

Compile on install is very rude. Please don't do this - npm install time is bad enough! For internal code where you want to avoid using an npm package repository, you can: 1) use a monorepo, 2) Upload and depend on npm pack output, 3) check in build output. – Coast

Answer 11

Deppending on the anatomy of your module, this solution may not work, but if your module is contained inside a single file, and has no dependencies (does not make use of import), using the following pattern you can release your code as it is, and will be able to be imported with import (Browser ES6 Modules) and require (Node CommonJS Modules)

As a bonus, it will be suittable to be imported using a SCRIPT HTML Element.

main.js :

(function(){
    'use strict';
    const myModule = {
        helloWorld : function(){ console.log('Hello World!' )} 
    };

    // if running in NODE export module using NODEJS syntax
    if(typeof module !== 'undefined') module.exports = myModule ;
    // if running in Browser, set as a global variable.
    else window.myModule = myModule ;
})()

my-module.js :

    // import main.js (it will declare your Object in the global scope)
    import './main.js';
    // get a copy of your module object reference
    let _myModule = window.myModule;
    // delete the the reference from the global object
    delete window.myModule;
    // export it!
    export {_myModule as myModule};

package.json :`

    {
        "name" : "my-module", // set module name
        "main": "main.js",  // set entry point
        /* ...other package.json stuff here */
    }

---

To use your module, you can now use the regular syntax ...

When imported in NODE ...

    let myModule = require('my-module');
    myModule.helloWorld();
    // outputs 'Hello World!'

When imported in BROWSER ...

    import {myModule} from './my-module.js';
    myModule.helloWorld();
    // outputs 'Hello World!'

Or even when included using an HTML Script Element...

<script src="./main.js"></script>
<script>
     myModule.helloWorld();
    // outputs 'Hello World!'
</script>

Cubicle