copy-mjs

Build tool to copy .js files from input directory to corresponding .mjs files in output directory.

Why?

If we have multiple files in a package and we want all of them to be importable we often suggest to users doing it like this:

import module from "package/lib/module";

This normally has a serious drawback because by doing so we often point people to using files authored in CommonJS format (being the lowest common denominator). This prevents bundlers' optimizations such as tree-shaking & scope hoisting (a.k.a. module concatenation).

If we provide a second build in ESM format we might mention it in the docs and propose importing the same thing with i.e. "package/es/module" for people using module-aware bundlers. This is a mental overhead that can be avoided.

I recommend using "proxy directories" technique described by me in this article as it allows wider set of tools to choose appropriate files automatically. I've also created a package which creates such directories for you - cherry-pick.

If you do not like this technique and you'd still like to improve the situation for your users this package might help you in that.

webpack@4 came with .mjs files support. What does it mean? It means that if your user imports "package/module" webpack will try to load package/module.mjs first, before falling back to loading package/module.js (tools not recognizing .mjs will fall back gracefully to .js). This means that we can just put .js files authored in CJS format side by side with .mjs files authored in ESM.

To provide the best developer experience you should either:

• publish alternate directory
• or move temporarily your files to root when publishing.

Doing so can allow people to import i.e. "package/module" instead of "package/lib/module".

CLI Options

copy-mjs

Copy `.js` files from input directory to `.mjs` files in output directory.

Options:
  --help, -h        Show help                                          [boolean]
  --version, -v     Show version number                                [boolean]
  --input-dir, -i                                               [default: "esm"]
  --output-dir, -o                                              [default: "lib"]
  --cwd                                                           [default: "."]

Example usage

copy-mjs --input-dir esm --output-dir lib

JS API

copy-mjs exports a default method which copies .js files from input directory to corresponding .mjs files in output directory. It accepts the same options as CLI command.

const copyMjs = require("copy-mjs");

copyMjs({ inputDir: "esm", outputDir: "lib" }).then(() =>
  console.log("Successfully created .mjs files.")
);