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).

Currently only for webpack 4 and above.

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: "development",
    entry: "index.js",
    output: {
        library: "LIB",
        libraryTarget: "var"
    },
    //...
    plugins: [
        new EsmWebpackPlugin()
    ]
}

Note 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.

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

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