Node.js Native ESM Support

:::note[New in v7.1.0] :::

Mocha supports writing your tests as ES modules, and not just using CommonJS. For example:

// test.mjs
import { add } from "./add.mjs";
import assert from "assert";

it("should add to numbers from an es module", () => {
  assert.equal(add(3, 5), 8);
});

To enable this you don't need to do anything special. Write your test file as an ES module. In Node.js this means either ending the file with a .mjs extension, or, if you want to use the regular .js extension, by adding "type": "module" to your package.json. More information can be found in the Node.js documentation.

Current Limitations

• Watch mode does not support ES Module test files
• Custom reporters and custom interfaces can only be CommonJS files
• Configuration file can only be a CommonJS file (.mocharc.js or .mocharc.cjs)
• Mocha in Node.js version 24.4.0 or older silently ignored top level errors in ESM files. If you cannot upgrade to a newer Node.js version, you can add --no-experimental-require-module to the NODE_OPTIONS environment variable.
• When using module-level mocks via libs like proxyquire, rewiremock or rewire, hold off on using ES modules for your test files. You can switch to using testdouble, which does support ESM.