esm-webpack-plugin

Output an ESM library from your bundle. Adds export statements to the end of the bundle for the exported members. Ideal for consumption by Javascript environments that support the ESM spec (aka: all major modern browsers).

IMPORTANT: This Plugin is currently only supported with Webpack 4.x.

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The purpose of this plugin was to provide a temporary workaround until support for ESM lands in Webpack, which at one point was targeted for v5.0 but unfortunately it did not land. My current understanding is that the Webapck team will continue to make progress in introducing full support for ESM output bundles at the v5.x level, but unclear what version it will actually land on. At this time, I don't have plans to release a version of this Plugin that supports Webpack v5.0, but if someone wants to contribute the necessary changes to make this compatible with v5.x, I will consider it 🙏 .

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Install

npm i -D @purtuga/esm-webpack-plugin

Usage

In your webpack configuration (webpack.config.js):

const EsmWebpackPlugin = require("@purtuga/esm-webpack-plugin");

module.exports = {
    mode: "production",
    entry: "index.js",
    output: {
        library: "LIB",
        libraryTarget: "var"
    },
    //...
    plugins: [
        new EsmWebpackPlugin()
    ]
}

Notice the use of output.library and output.libraryTarget, which indicates a library is being built and the bundle will expose it via a scoped variable named LIB.

NOTE: the value for output.library should NOT match the name of an exported library member.

If using this plugin on a CommonJS source project, see the FAQ below for more information.

Options

Options are supported by providing an object to the Plugin constructor. All are optional. Example:

const EsmWebpackPlugin = require("@purtuga/esm-webpack-plugin");

module.exports = {
    //...
    plugins: [
        new EsmWebpackPlugin({
            /*... Plugin Options here ...*/
        })
    ]
}

Supported options:

• exclude {Function}: A callback function that will be used to determine if a given file name (a named output file) should be excluded from processing. By default, all files whose file extension does not end with .js or .mjs will be excluded (meaning: no ESM export statements will be added to the output file). Note that callback is applied to the named output chunks that webpack outputs. Function callback will receive two arguments - the fileName that is being process and webpack's chunk object that contains that file name.

new EsmWebpackPlugin({
    exclude(fileName, chunk) {
        // exclude if not a .js/.mjs/.cjs file
        return !/\.[cm]?js/i.test(fileName);
    }
})

• skipModule {Function}: A callback function that can be used to skip over certain modules whose exports should not be included. Useful for when certain development plugins from webpack are used (like the devServer). The callback is provided with two arguments - the file name for the given module and the Webpack module class instance. Example - don't include webpack devServer generated bundles and modules:

new EsmWebpackPlugin({
    exclude(fileName) {
        // Exclude if:
        //  a. not a js file
        //  b. is a devServer.hot file
        return !/\.[cm]?js$/i.test(fileName) ||
            /\.hot-update\.js$/i.test(fileName);
    },
    skipModule(fileName, module) {
        return /[\\\/]webpack(-dev-server)?[\\\/]/.test(moduleName);
    }
})

• moduleExternals {boolean}: A boolean that determines whether webpack externals should be imported as ES modules or not. Defaults to false. When set to true, the defined webpack externals will be added to the output ES module as import's. Example: Given the following code module

import foo from 'foo-mod'

export const doFoo = () => foo();

with a webpack configuration containing the following:

const EsmWebpackPlugin = require("@purtuga/esm-webpack-plugin");

module.exports = {
    //...
    externals: {
        'foo-mod': '/some/external/location/foo-mod.js'
    },
    plugins: [
        new EsmWebpackPlugin({
            /*... Plugin Options here ...*/
        })
    ]
}

would generate an ESM with the following import:

import * as __WEBPACK_EXTERNAL_MODULE__0__ from '/some/external/location/foo-mod.js';

var LIB =
/******/ (function(modules) { // webpackBootstrap
//...
})();
//...
export {
    _LIB$doFoo as doFoo
}

• esModuleExternals {boolean}: This option applies only when moduleExternals options is true (see above). A boolean that determines whether esm-webpack-plugin will add the __esModule property to all imported externals. This can be helpful for improving interop between CJS and ESM modules, since webpack treats modules with the __esModule property differently than modules without them. Defaults to true.

  To add the __esModule property, esm-webpack-plugin uses a function cloneWithEsModuleProperty() which creates a new object that proxies to the original module, since ES modules are not extensible.

Example

Given the above Usage example:

Entry File: index.js

import {horn} from "lib/noises"
export {bark} from "lib/noises"

export function makeHornNoise() {
    horn();
}

export default makeHornNoise;

Library module lib/noises.js

export function horn() {
    return "honk honk";
}

export function bark() {
    return "woof woof";
}

Output Bundle

var LIB = (/******/ (function(modules){/* webpack bundle code */}));

export const bark = LIB['bark'];
export const makeHornNoise = LIB['makeHornNoise'];
export default LIB['default'];

Example of usage on the Browser

In the browser:

<script type="module">
    import MyLibrary from "MyLibrary.js";
    MyLibrary(); // makeHornNoise
</script>

Or:

<script type="module">
    import {bark, makeHornNoise} from "MyLibrary.js"
    bark();
    makeHornNoise();
</script>

FAQ

When using the generated ESM library, un-used exports are not removed from final output (not three-shaken)

This is, unfortunately, a drawback and limitation of this plugin. This plugin does not change how the code is bundled or structured by webpack and only adds export statements to the end of file in order to enable its use via ES6 import. Because of that, tree-shaking is not possible - all code is already bundled and stored using webpack's internal structure. The ability to possibly support tree-shaking can only truly be supported when webpack itself introduces support for generating ESM output targets.

My advice is to use the generated ESM modules at runtime when no build/bundling pipeline exists on a project and to import source directly (if that is coded as ESM) when a pipeline does exists.

With CommonJS project, individual exports are not available in the output ESM module

This project was created primarily for use in sources that are developed using ESM. The default behavior, if the plugin is unable to identify explicit export's is to expose the entire library object (the LIB variable as seen in the examples above). A workaround that might work is to create an ESM entry source file whose sole purpose is to expose the desired members and use that as your webpack entry file. Here is an example:

File /index.cjs:

exports.libA = require("./lib-a.cjs").libA;
exports.cjsIndex = function cjsIndex() {
    console.log("src-cjs/index.cjs loaded!");
}

File /index.mjs (use this with webpack):

import * as cjs from "./index.cjs";

const { libA, cjsIndex } = cjs;

export default cjs;
export {
    libA,
    cjsIndex
};

Note that in order for this work, I believe (have not confirmed) that webpack's mode needs to be javascript/auto which I think is currently the default.

Uncaught SyntaxError: Identifier 'MyLibrary' has already been declared

Where MyLibrary is the same name as the output.library value in your webpack.config.js file.

This occurs when your library exports a member that is named the same as the value found in the output.library value. It is suggested that you use an obscure value for output.library - one that has low probability of matching an exported member's name.

TypeError: chunk.entryModule.buildMeta.providedExports.reduce is not a function

Console output:

/home/prj/node_modules/@purtuga/esm-webpack-plugin/esm-webpack-plugin.js:45
    chunk.entryModule.buildMeta.providedExports.reduce((esm_exports, exportName) => {
                                                ^

TypeError: chunk.entryModule.buildMeta.providedExports.reduce is not a function

In order to create an ESM package, webpack must be able to identify your module exports. This error is likey due to the fact that it was not able to do that. You can run your build with --bail --display-optimization-bailout to see if the following message is output against your entry module: ModuleConcatenation bailout: Module exports are unknown

The root cause is likely due to exporting modules using the * syntax where different modules have an export named exactly the same. Example:

index.js

export * from "mod1.js";
export * from "mod1.js"

Where both modules have an export name foo. To address this issue, try using named exports instead:

export {foo} from "mod1.js";
export {foo as foo2} from "mod2.js"

License

MIT