@skylernpm/quasi-est-modi

Babel plugin to replace extension of file name written in import statement and dynamic import.

Installation

npm install --save-dev @skylernpm/quasi-est-modi

Example

With the option:

{ "extMapping": { ".js": ".mjs" }}

In

import { foo } from './module1.js';
export { bar } from './module2.js'; // Works for re-exporting
const promise = import('./module3' + '.js'); // Also works for dynamic import!

Out

import { foo } from './module1.mjs';
export { bar } from './module2.mjs';

// In dynamic import, function to replace extension is inserted.
// Note the actual code is not exactly the same.
const promise = import(__transformExtension('./module3' + '.js'));

Why We Need This Plugin?

When you develop a npm package that includes both ESModule and CommonJS version of the code, there is two ways to tell Node which file is which version.

• Distinguish files by their extension, mjs for ESModule and cjs for CommonJS.
• If two versions are located in separate directories, put a package.json with a type field specified to the directory.

If you choose the former and you write your code in ESModule and transpile it to CommonJS, you have to change the extension of the files while transpiling.

In Babel CLI, extension of the output file name can be changed with --out-file-extension option. But the file name referenced inside the code is not changed. In this case, this plugin comes into play.

Note that the conversion is performed only on relative file name (starts with ./ or ../), because built-in packages or packages importing from node_modules should not be converted.

Usage

If project root package.json has type field of module, Babel config of

{
  "plugins": [
    ["replace-import-extension", { "extMapping": { ".js": ".cjs" }}],
    ["@babel/transform-modules-commonjs"]
  ]
}

will convert the file extension from .js to .cjs and convert ESModule to CommonJS, allowing both version's code exist together while Node can handle each versions correctly. (@babel/plugin-transform-modules-commonjs must be installed.) Or if you also need other translations, @babel/env preset can be used together like,

{
  "presets": [["@babel/env"]],
  "plugins": [
    ["replace-import-extension", { "extMapping": { ".js": ".cjs" }}]
  ]
}

If project root package.json has no type field or has type field of cjs, ESModule files must be explicitly marked by mjs extension, which can be done by Babel config of

{
  "plugins": [
    ["replace-import-extension", { "extMapping": { ".js": ".mjs" }}]
  ]
}

Once again, --out-file-extension option must be used together to change the output file extension.

Supporting both mjs and cjs in the same package

If you are using .mjs for your source files, you can use babel to generate .cjs files for backwards compatibility:

{
  "presets": [["@babel/env"]],
  "plugins": [
    ["replace-import-extension", { "extMapping": { ".mjs": ".cjs" }}]
  ]
}

In your package.json specify the entries accordingly:

{
  "main": "dist/index.cjs",
  "module": "src/index.mjs",
  "source": "src/index.mjs",
  "exports": {
    ".": {
      "require": "dist/index.cjs",
      "import": "src/index.mjs"
    },
    "src/index.mjs": {
      "import": "src/index.mjs"
    },
    "dist/index.cjs": {
      "require": "dist/index.cjs",
      "import": "dist/index.cjs"
    }
  },
  "scripts": {
    "build.cjs": "babel -d dist/ src/ --out-file-extension .cjs",
    "prepare": "npm run build.cjs"
  }
}

Options

extMapping

Object, defaults to {}.

Mapping of original extension to converted extension. Leading . is mandatory.

Both the original and the converted extensions can be empty string '', which means no extension. You can use this feature to add or remove extension.