Jest Hoisting Bug Proof of Concept for .cjs files

Setup Instructions

1. Clone the repository:

   git clone git@github.com:klocke-io/jestHoistingTest.git
   cd jestHoistingTest

2. Install dependencies:

   npm install

3. Run specific test files: For the .js test: (Works as expected ✅)

   npx jest __tests__/index.spec.js

   For the .cjs test: (Dose not work as expected ❌)

   npx jest __tests__/index.spec.cjs

   For the manually hoisted .cjs test (Works as expected ✅):

   npx jest __tests__/index.manually-hoisted.cjs

Project and Code Explanation

This repository demonstrates a potential bug in Jest related to the hoisting of jest.mock when using .cjs file extension for test files.

• lib/index.js: Exports a function businessMessage that returns 'foo bar'.

  // lib/index.js
  function businessMessage() {
    return 'foo bar';
  }
  module.exports = { businessMessage };

• tests/index.spec.js: ✅ Standard Jest test in CommonJS, with jest.mock after the require. This works as expected because Jest hoists the mock.

  // __tests__/index.spec.js
  const { businessMessage } = require('../lib');
  jest.mock('../lib', () => ({
    businessMessage: jest.fn(() => 'mocked!'),
  }));
  
  test('should use the mocked implementation', () => {
    expect(businessMessage()).toBe('mocked!');
  });

• tests/index.spec.cjs: ❌ Identical to the .js test, but with a .cjs extension. Here, jest.mock is not hoisted, so the real implementation is used instead of the mock.

  // __tests__/index.spec.cjs
  const { businessMessage } = require('../lib');
  jest.mock('../lib', () => ({
    businessMessage: jest.fn(() => 'mocked!'),
  }));
  
  test('should use the mocked implementation', () => {
    expect(businessMessage()).toBe('mocked!');
  });
  // This test will fail because jest.mock is not hoisted in .cjs files

• tests/index.manually-hoisted.cjs: ✅ The mock is placed before the require statement, manually simulating hoisting. This test works as expected, even with the .cjs extension.

  // __tests__/index.manually-hoisted.cjs
  jest.mock('../lib', () => ({
    businessMessage: jest.fn(() => 'mocked!'),
  }));
  const { businessMessage } = require('../lib');
  
  test('should use the mocked implementation', () => {
    expect(businessMessage()).toBe('mocked!');
  });

• jest.config.cjs: Configures Jest to pick up both .js and .cjs test files.

  // jest.config.cjs
  module.exports = {
    testMatch: [
      '**/__tests__/**/*.js',
      '**/__tests__/**/*.cjs',
    ],
  };

Key Point

• Jest's automatic hoisting of jest.mock does not work in .cjs files, but works in .js files. Manual hoisting (placing jest.mock before require) is required for .cjs files.

Test Environment

• Node.js version: v23.10.0
• npm version: 10.9.2
• npx version: 10.9.2
• Jest version: 29.7.0
• OS: macOS 15.5 (Darwin Kernel Version 24.5.0, arm64)

Command:

node --version
npm --version
npx --version
npx jest --version
uname -a
sw_vers

Output:

v23.10.0
10.9.2
10.9.2
29.7.0
Darwin LQ9357DK4K 24.5.0 Darwin Kernel Version 24.5.0: Tue Apr 22 19:53:27 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6041 arm64
ProductName:            macOS
ProductVersion:         15.5
BuildVersion:           24F74