Updating from CommonJS to ESM

If your Node.js project uses CommonJS modules (e.g. it uses require), you'll need to update your code to use ECMAScript modules (ESM) to run it in Deno. This guide will help you update your code to use ESM syntax.

Module imports and exports Jump to heading#

Deno supports ECMAScript modules exclusively. If your Node.js code uses require, you should update it to use import statements instead. If your internal code uses CommonJS-style exports, those will also need to be updated.

A typical CommonJS-style project might look similar to this:

module.exports = function addNumbers(num1, num2) {
  return num1 + num2;
};

const addNumbers = require("./add_numbers");
console.log(addNumbers(2, 2));

To convert these to ECMAScript modules, we'll make a few minor changes:

export function addNumbers(num1, num2) {
  return num1 + num2;
}

import { addNumbers } from "./add_numbers.js";
console.log(addNumbers(2, 2));

Exports:

Imports:

Quick fix with VS Code Jump to heading#

If you are using VS Code, you can use its built-in feature to convert CommonJS to ES6 modules. Right-click on the require statement, or the lightbulb icon and select Quick Fix and then Convert to ES module.

CommonJS vs ECMAScript resolution Jump to heading#

An important distinction between the two module systems is that ECMAScript resolution requires the full specifier including the file extension. Omitting the file extension, and special handling of index.js, are features unique to CommonJS. The benefit of the ECMAScript resolution is that it works the same across the browser, Deno, and other runtimes.

🦕 Now that you know how to port from CJS to ESM you can take advantage of the modern features that ESM offers, such as async module loading, interop with browsers, better readability, standardization and future proofing.