Test runner module mocking doesn't work for CJS if package exports are specified

[sloops77]

Version

22.15

Platform

Darwin Andress-MacBook-Pro.local 24.4.0 Darwin Kernel Version 24.4.0: Fri Apr 11 18:33:47 PDT 2025; root:xnu-11417.101.15~117/RELEASE_ARM64_T6000 arm64

Subsystem

node:mock

What steps will reproduce the bug?

The issue is for packages that expose esm module and cjs main. See Repo: https://github.com/sloops77/node-mock-bug for more examples

Rather than create my own package the examples below use the nanoid 3.x package. The linked repo also shows that it is reproducible for the ethers package (which also exposes module and main).

// snippet of package.json from nanoid
{
  "name": "nanoid",
  "version": "3.3.11",
  "description": "A tiny (116 bytes), secure URL-friendly unique string ID generator",
  "type": "module",
  "main": "index.cjs",
  "module": "index.js",
  ...

// test.js
const {describe, it, mock} = require("node:test");
const assert = require("node:assert");

mock.module('nanoid', {
  namedExports: {
    nanoid() {
      return '1234'
    }
  },
});


test('mocking problem in cjs', () => {
  const {nanoid} = require('nanoid')
  assert.strictEqual(nanoid(), '1234'); // fails test because cjs is not mocked
})

test('mocking works for esm', async () => {
  const {nanoid} = await import('nanoid')
  assert.strictEqual(nanoid(), '1234'); // works as it should
})

How often does it reproduce? Is there a required condition?

1. For imported packages it seems 100% consistent. It also occurs for a module mock that uses defaultExport.
2. Node core modules do not reproduce this issue

What is the expected behavior? Why is that the expected behavior?

CJS & MJS files should receive a mocked module

What do you see instead?

CJS files receive the original module, not the mock. MJS files are fine as they receive the mock.

Additional information

No response

[sloops77]

Seems related #53634

[ljharb]

The "module" field is nonstandard and node doesn't look at it, afaik. Does the same thing happen if you just delete the module field?

[sloops77]

@ljharb
You are correct - after some experimentation is seems related to the use of exports

The example package should be installed into node_modules.

// index.cjs
const add = (x, y) => x+y;

module.exports = {add}

// index.js
export const add = (x, y) => x+y;

{
  "name": "example-package",
  "type": "module",
  "main": "index.js",
  "exports": {
    "default": {
      "import": "./index.js",
      "require": "./index.cjs"
    }
  },
  "private": true
}

// test.js
mock.module('example-package', {
  namedExports: {
    add(x, y) {
      return 1+x+y;
    }
  },
});

describe('example-package', () => {
  it('mocking problem in cjs', () => {
    const {add} = require('example-package')
    assert.strictEqual(add(4,5), 10); // fails
  })

  it('mocking works for esm', async () => {
    const {add} = await import('example-package');
    assert.strictEqual(add(4,5), 10);
  })
})

Remove exports from package.json and the tests (and mock) works as expected

[sloops77]

@ljharb this is a pretty common pattern across many npm libraries. It is also affecting https://github.com/remix-run/react-router v7

[sloops77]

@JakobJingleheimer @cjihrig I would like to get your thoughts on this issue thx

[JakobJingleheimer]

Hiya! Could you try setting defaultExport to anything (ex an empty object).

[joyeecheung]

My guess is that the test runner mocking utility uses the async loader hooks to mock CJS and uses a source text module facade for this, which ends up relying on the export detection using the static analysis that does not recognize patterns like module.exports = { add }. If changing it to module.exports.add = add works then it would be that.

EDIT: actually cjs-module-lexer should recognize module.exports = { add }, so it's probably not that.

[JakobJingleheimer]

I'll be doing some work related to that in the next couple weeks, so if it is similar to that, I'll address it at the same time (it would happen to also fix this).

[sloops77]

@JakobJingleheimer @joyeecheung

I tried setting both your suggestions. Neither fixed the issue

[sloops77]

@JakobJingleheimer should this issue get the test_runner label?