Custom esm loader

Custom loader for ESM modules. You need node 20 or upper.

Why?

We know that ESM Modules are very different to CommonJs and one of the things that changes too much is the imports's path. For example:

import something from './file' // this won't work

This import path will work in CommonJs but in ESM no. The correct form is:

import something from './file.js' // this will work

Some parsers like Typescript don't follow well this standard, and for some editors is uncorfortable to change the imports to add the .js extension. Also, the directories import in ESM not exist. For example:

// src/module/index.js
export const name = 'module'

// src/main.js
import { name } from './module' // this won't work

Again in CommonJs this import path will work, but in ESM no. The correct form is:

// src/main.js
import { name } from './module/index.js' // this will work

Both cases could be very confused and uncomfortable for many developers, because in environments like Vite, webpack, swc, Typescript, and others is not necessary to add .js or specify the index.js.

Another problem when you use ESM Modules is the definitions of __dirname, __filename, require. This global variables exist on CommonJs, but in ESM no. If you try to get one of this varaibles, you will get an error like this:

ReferenceError: __dirname is not defined in ES module scope
This file is being treated as an ES module because it has a '.js' file extension and 'E:\Daniel-Pc\Dev-Pro\nodejs\test-loader-import-\package.json' contains "type": "module". To treat it as a CommonJS script, rename it to use the '.cjs' file extension.
    at file:///E:/Daniel-Pc/Dev-Pro/nodejs/test-loader-import-/dist/main.js:3:13
    at ModuleJob.run (node:internal/modules/esm/module_job:218:25)
    at async ModuleLoader.import (node:internal/modules/esm/loader:329:24)
    at async loadESM (node:internal/process/esm_loader:28:7)
    at async handleMainPromise (node:internal/modules/run_main:113:12)

Node.js v20.11.0

How?

For that reason is possible to modify the path resolver to solvent this problem and use ESM with more flexibility. With this loader you don't have to set .js and index.js to imports, node can load all of this paths. And you can use __dirname, __filename and require. You only have to modify the script for run the project (in the example you can see how you have to set it). If you apply the loader to previous examples:

Single file import:

import something from './file' // this will work with the loader

Directory import:

// src/main.js
import { name } from './module' // this will work with the loader

Internal node valiables:

// All of this variables work with the loader, you don't have to set it manually
__dirname
__filename
require

// Also import.meta will include it
import.meta.filename // The same of __filename
import.meta.dirname // The same of __dirname

NOTE

This variables would be created in the project files, third libraries won't be modified.

Installation

Use the package manager npm to install custom-esm-leader.

npm i esm-import-loader

or by pnpm

pnpm add esm-import-loader

Usage

You must to put on your start script like this:

node --import esm-import-loader <entry file>

Contributing

Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change.

Please make sure to update tests as appropriate.

License

MIT