Jest ModuleNameMapper: Unexpected Token CJS Folder Error Fix

The Jest ModuleNameMapper is a powerful tool for mapping module names to their corresponding paths in a project. However, it can sometimes throw an "Unexpected Token CJS Folder" error. This error typically occurs when Jest encounters a CommonJS (CJS) folder that it doesn't know how to handle.

Understanding the Error

The “Unexpected Token CJS Folder” error usually arises from a mismatch between the expected module system (ESM) and the actual module system (CJS) used in a project. Jest, by default, supports both ESM and CJS, but it can get confused when it encounters a CJS folder in an ESM project or vice versa.

Causes of the Error

There are several reasons that might lead to this error:

• Inconsistent module systems: Mixing ESM and CJS in a project can cause this error.
• Incorrect configuration: Misconfiguring the moduleNameMapper or moduleFileExtensions in the Jest configuration file can lead to this issue.
• Third-party library issues: Some third-party libraries might not be compatible with Jest’s default module system.

Fixing the Error

To fix the “Unexpected Token CJS Folder” error, you can try the following solutions:

Solution 1: Configure ModuleNameMapper

You can configure the moduleNameMapper to handle CJS folders explicitly. Here’s an example:

// jest.config.js
module.exports = {
  moduleNameMapper: {
    '^some-cjs-folder/(.*)$': '<rootDir>/some-cjs-folder/$1',
  },
};

This configuration maps the some-cjs-folder to its actual path in the project.

Solution 2: Update ModuleFileExtensions

You can also update the moduleFileExtensions to include js and cjs:

// jest.config.js
module.exports = {
  moduleFileExtensions: ['js', 'cjs', 'json'],
};

This configuration tells Jest to handle both js and cjs files.

Solution 3: Use the transform option

You can use the transform option to transform CJS files to ESM:

// jest.config.js
module.exports = {
  transform: {
    '^.+\\.cjs$': 'babel-jest',
  },
};

This configuration uses Babel to transform CJS files to ESM.

Best Practices

To avoid this error in the future, follow these best practices:

• Ensure consistency in your project’s module system.
• Configure Jest to handle different module systems.
• Test your project thoroughly to catch any module system-related issues.