How to include commonjs module in ES6 module node app?

Question

I have a node app that I'd like to use in the standard ES6 module format (i.e., "type": "module" in the package.json, and using import and export throughout) without transpiling down to ES5. But I would like to take advantage of some older libraries such as a express and socket.io that use CommonJS / require format. What are my options (as of 5/2020, Node 12.16.3) for combining CommonJS modules into an ES6 app?

Answer 1

Working with CommonJS modules is pretty straight forward. You can only do default exports from CommonJS modules.

import packageMain from 'commonjs-package'; // Works
import { method } from 'commonjs-package'; // Errors

This means that all commonjs exports will live on the packageMain object, and you need to dot in to the packageMain object to pickup what you need.

packageMain.method1()

More info in the official nodejs docs

Answer 2

Since Node 13.10, there is another option, the most forward-looking one:

File an issue in the repo of the CommonJS library you'd like to use, persuading the maintainers to publish dual packages (ESM + CommonJS), using conditional exports.

For libraries written in TypeScript, generating dual packages is easy, and doesn't require Babel or rollup or any additional tools. Here's how I did it in local-iso-dt:

Relevant parts of package.json:

{
  "name": "local-iso-dt",
  "version": "3.1.0",
  "description": "...",
  "type": "commonjs",
  "exports": {
    "node": {
      "import": "./index.mjs",
      "require": "./index.js"
    },
    "default": "./index.mjs"
  },
  "main": "index.js",
  "files": [
    "index.ts",
    "index.mjs",
    "index.js"
  ],
  "scripts": {
    "clean": "rm index*.js index.mjs",
    "prepublishOnly:cjs": "tsc index.ts --esModuleInterop --removeComments",
    "prepublishOnly:esm": "tsc index.ts -t ES2015 --types node && mv index.js index.mjs",
    "prepublishOnly": "npm run prepublishOnly:esm; npm run prepublishOnly:cjs"
  },
  "devDependencies": {
    "typescript": "^4.0.2"
  },
}

prepublishOnly:esm renames the output manually because TypeScript can't yet generate .mjs output directly and --outFile doesn't work with ES Modules.

The exports block has the "conditional exports that enable TypeScript code transpiled with ES Modules, to use named imports. TypeScript doesn't directly support .mjs input files.

No tsconfig.json was necessary for this simple module.

Answer 3

Ivan's answer is helpful, but I also should note that that because I was using node 12.16.3, I also needed to add the --experimental-modules flag to my start script in package.json:

"type": "module",
"scripts": {
    "start": "node --experimental-modules --experimental-json-modules server.mjs",
}

• "type": "module" tells node that you should use ES6 modules
• file name of .mjs further makes it clear that we're working with a module JavaScript file
• --experimental-modules and --experimental-json-modules let you use import in node version 12.

Then I was able to do things like import express from 'express'; (where express 4.17.1 is a CommonJS module), and it worked fine.

Answer 4

Make sure the module in question has a index.js that exports the code you want to import or require.

In the package.json under the dependencies attribute add the following:

"myCustomModule":"file:./path/to/myCustomModule"

After you have done this run npm install.

Once that is completed look inside the node_modules of the project root and you will see a directory named myCustonModule.

Now in any project file you can import or require that code as you would any other mode_module