This package has been deprecated

Author message:

This library has been renamed. Please use cjstoesm instead: https://github.com/wessberg/cjstoesm/

@wessberg/cjs-to-esm-transformer

A Custom Transformer for Typescript that transforms Node-style CommonJS to tree-shakeable ES Modules

Description

This is a Custom Transformer for Typescript that converts Node-style CommonJS modules into treeshakeable ES Modules. This allows you to not only bundle CommonJS modules for the browser, but also to bundle them in modern tools such as Rollup.

Prior art such as babel-plugin-transform-commonjs and rollup-plugin-commonjs exists, but this Custom Transformer aims at producing code that is just as tree-shakeable as equivalent code written natively with ES Modules. Additionally, it aims to be as clean as possible, with no "wrappers" around modules as can be seen in other similar solutions.

This has been implemented as a TypeScript Custom Transformer since:

• Typescript is a superset of JavaScript, and as such:
• This works with both JavaScript and TypeScript
• This is intentionally so low-level that it can be used as the underlying implementation in anything you want, whether it be directly with Typescript's Compiler APIs, Webpack loaders, Rollup plugins, CLIs, or even linting tools.

For example, here's how this Custom Transformer may rewrite a CommonJS module:

Input

exports.foo = function foo() {};

Output

export function foo() {}

Here's another example:

Input

module.exports = {
	foo() {
		return 2 + 2;
	},
	bar: 3,
	baz: new RegExp("")
};

Output

export function foo() {
	return 2 + 2;
}
export const bar = 3;
export const baz = new RegExp("");
export default {foo, bar, baz};

The same goes for require(...) calls:

Input:

const {foo: bar} = require("./my-module");

Output:

import {foo as bar} from "./my-module";

And for complex require calls such as:

Input:

const {
	foo: {bar: baz}
} = require("./my-module").something("bar");

Output:

import {something} from "./my-module";
const {
	foo: {bar: baz}
} = {something}.something("bar");

As you can see, this transformer will attempt to produce code that generates as granular imports and exports as possible.

Features

• Transformation of CommonJS to ESM
• Tree-shaking friendly
• Clean, idiomatic output
• No wrappers
• Low-level implementation that can be used as the foundation for other tools such as Loaders, Plugins, CLIs, and Linters.
• CLI integration, enabling you to convert a project from CJS to ESM from the command line.

Table of Contents

• Description
  • Features
• Table of Contents
• Install
  • npm
  • Yarn
  • pnpm
  • Run once with npx
  • Peer Dependencies
• Usage
  • Usage with TypeScript's Compiler APIs
  • Usage with Rollup
    • Usage with rollup-plugin-ts
    • Usage with rollup-plugin-typescript2
  • Usage with Webpack
    • Usage with awesome-typescript-loader
    • Usage with ts-loader
• CLI
• Options
• Contributing
• Maintainers
• Backers
  • Patreon
• FAQ
  • Is conditional require(...) syntax converted into dynamic imports?
• License

Install

npm

$ npm install @wessberg/cjs-to-esm-transformer

Yarn

$ yarn add @wessberg/cjs-to-esm-transformer

pnpm

$ pnpm add @wessberg/cjs-to-esm-transformer

Run once with npx

$ npx -p typescript -p @wessberg/cjs-to-esm-transformer cjstoesm

Peer Dependencies

@wessberg/cjs-to-esm-transformer depends on typescript, so you need to manually install these as well.

Usage

Since this is a Custom Transformer, it can be used practically anywhere you use TypeScript.

This Custom Transformer works completely fine on JavaScript files, so long as you enable allowJs in your CompilerOptions.

The most obvious place would be to use it directly with Typescript's compiler APIs:

Usage with TypeScript's Compiler APIs

The most simple way of transpiling with Typescript would be with transpileModule:

import {ModuleKind, transpileModule} from "typescript";
import {cjsToEsm} from "@wessberg/cjs-to-esm-transformer";

const result = transpileModule(`const {join} = require("path");`, {
	transformers: cjsToEsm(),
	compilerOptions: {
		module: ModuleKind.ESNext
	}
});

// 'import { join } from "path"' is printed to the console
console.log(result.outputText);

You may use this is conjunction with other Custom Transformers by importing commonJsToEsmTransformerFactory instead:

import {ModuleKind, transpileModule} from "typescript";
import {cjsToEsmTransformerFactory} from "@wessberg/cjs-to-esm-transformer";

transpileModule(`const {join} = require("path");`, {
	transformers: {
		before: [cjsToEsmTransformerFactory(), someOtherTransformerFactory()],
		after: [
			// ...
		],
		afterDeclarations: [
			// ...
		]
	},
	compilerOptions: {
		module: ModuleKind.ESNext
	}
});

You can also use Custom Transformers with entire Typescript Programs:

import {getDefaultCompilerOptions, createProgram, createCompilerHost} from "typescript";
import {cjsToEsm} from "@wessberg/cjs-to-esm-transformer";

const options = getDefaultCompilerOptions();
const program = createProgram({
	options,
	rootNames: ["my-file.js", "my-other-file.ts"],
	host: createCompilerHost(options)
});
program.emit(undefined, undefined, undefined, undefined, cjsToEsm());

Usage with Rollup

There are two popular TypeScript plugins for Rollup that support Custom Transformers:

• rollup-plugin-ts
• rollup-plugin-typescript2

Usage with rollup-plugin-ts

import ts from "@wessberg/rollup-plugin-ts";
import {cjsToEsm} from "@wessberg/cjs-to-esm-transformer";

export default {
	input: "...",
	output: [
		/* ... */
	],
	plugins: [
		ts({
			transformers: [cjsToEsm()]
		})
	]
};

Usage with rollup-plugin-typescript2

import ts from "rollup-plugin-typescript2";
import {cjsToEsm} from "@wessberg/cjs-to-esm-transformer";

export default {
	input: "...",
	output: [
		/* ... */
	],
	plugins: [
		ts({
			transformers: [() => cjsToEsm()]
		})
	]
};

Usage with Webpack

There are two popular TypeScript loaders for Webpack that support Custom Transformers:

• awesome-typescript-loader
• ts-loader

Usage with awesome-typescript-loader

import {cjsToEsm} from "@wessberg/cjs-to-esm-transformer";
const config = {
	// ...
	module: {
		rules: [
			{
				// Match .mjs, .js, .jsx, and .tsx files
				test: /(\.mjs)|(\.[jt]sx?)$/,
				loader: "awesome-typescript-loader",
				options: {
					// ...
					getCustomTransformers: () => cjsToEsm()
				}
			}
		]
	}
	// ...
};

Usage with ts-loader

import {cjsToEsm} from "@wessberg/cjs-to-esm-transformer";
const config = {
	// ...
	module: {
		rules: [
			{
				// Match .mjs, .js, .jsx, and .tsx files
				test: /(\.mjs)|(\.[jt]sx?)$/,
				loader: "ts-loader",
				options: {
					// ...
					getCustomTransformers: () => cjsToEsm
				}
			}
		]
	}
	// ...
};

CLI

You can also use this library as a CLI to convert your project files from using CommonJS to using ESM. This is still considered somewhat experimental. If you have any issues, please submit an issue.

If you install @wessberg/cjs-to-esm-transformer globally, you'll have cjstoesm in your path. If you install it locally, you can run npx cjstoesm. If you don't install it globally nor locally, you can run it with npx with the command npx -p typescript -p @wessberg/cjs-to-esm-transformer cjstoesm <glob> <outdir>.

$ cjstoesm --help

Welcome to the CJS to ESM CLI!

Usage: cjstoesm [options] [command]

Options:
  -h, --help                            output usage information

Commands:
  transform [options] <input> <outDir>  Transforms CJS to ESM modules based on the input glob

For example, you can run cjstoesm transform "**/*.*" dist to transform all files matched by the glob **/*.* and emit them to the folder dist from the current working directory. You can also just run cjstoesm "**/*.*" dist which is an alias for the transform command.

Options

You can provide options to the cjsToEsm Custom Transformer to configure its behavior:

Option	Description
debug (optional)	If true, errors will be thrown if unexpected or unhandled cases are encountered. Additionally, debugging information will be printed during transpilation.
readFile (optional)	A function that will receive a file name and encoding and must return its string contents if possible, and if not, return undefined.
fileExists (optional)	A function that will receive a file name and must return true if it exists, and false otherwise
typescript (optional)	If given, the TypeScript version to use internally for all operations.

Contributing

Do you want to contribute? Awesome! Please follow these recommendations.

Maintainers

Frederik Wessberg Twitter: @FredWessberg Github: @wessberg Lead Developer

Backers

Bubbles Twitter: @use_bubbles

Patreon

FAQ

Is conditional require(...) syntax converted into dynamic imports?

No. For the input:

const result = true ? require("./foo") : require("./bar");

The following may be the output, depending on the internal structure of the modules referenced by the require calls:

import foo from "./foo";
import bar from "./bar";

const result = true ? foo : bar;

CommonJS require() syntax are Expressions, whereas ESM import/export syntax are Declarations, and to achieve the same expressiveness with ESM, dynamic imports are required. However, these return Promises and as such cannot be transformed equivalently.

License

MIT © Frederik Wessberg (@FredWessberg) (Website)