Jest and ESM: Cannot use import statement outside a module

When working with Jest and ECMAScript Modules (ESM), you may encounter the error message “Cannot use import statement outside a module.” This error typically occurs when Jest encounters an import statement in a file that is not recognized as an ECMAScript module.

To resolve this issue and enable Jest to handle ECMAScript module imports, you can follow these steps:

Step 1: Configure Jest to Support ESM

1. Install the @babel/preset-env package as a development dependency:

npm install --save-dev @babel/preset-env

2. Create a .babelrc file in the root of your project and add the following configuration:

{
  "presets": ["@babel/preset-env"]
}

3. Install the esm package as a development dependency:

npm install --save-dev esm

4. Create a jest.config.js file in the root of your project and add the following configuration:

module.exports = {
  transform: {},
  transformIgnorePatterns: [],
  setupFilesAfterEnv: ["esm"],
};

Step 2: Update Your Test Files

1. Open your test files and change the file extension from .js to .mjs. For example, if you have a test file named example.test.js, rename it to example.test.mjs.
2. Update the import statements in your test files to use the .mjs extension:

import { someFunction } from './path/to/module.mjs';

Step 3: Run Jest with ESM Support

Run Jest using the --experimental-vm-modules flag to enable support for ECMAScript modules:

npx jest --experimental-vm-modules

Jest should now be able to handle import statements in your test files and properly execute tests that rely on ECMAScript modules.

Note: It’s important to ensure that your project’s dependencies and configurations are set up correctly. Make sure you have installed the necessary packages and configured Jest accordingly.

By following these steps, you can resolve the “Cannot use import statement outside a module” error and use import statements in your Jest tests when working with ECMAScript modules.

If you’re still encountering the “Cannot use import statement outside a module” error while using Jest and ECMAScript Modules (ESM), there are a few additional troubleshooting steps you can try:

Step 1: Check Node.js Version

Ensure that you are using a version of Node.js that supports ECMAScript modules. ECMAScript modules were introduced in Node.js version 12 and later. If you are using an older version of Node.js, consider upgrading to a newer version.

Step 2: Verify File Extensions

Double-check that your test files have the correct file extensions. Jest expects test files to have a .test.js or .spec.js extension by default. If you have renamed your test files to .mjs as suggested earlier, make sure you update the Jest configuration accordingly.

Step 3: Update Jest Configuration

If the previous steps did not resolve the issue, you can try updating your Jest configuration to explicitly handle ECMAScript modules.

In your jest.config.js file, add the following transformation configuration:

module.exports = {
  transform: {
    '^.+\\.mjs$': 'babel-jest',
  },
  transformIgnorePatterns: [],
  setupFilesAfterEnv: ['esm'],
};

This configuration tells Jest to use the babel-jest transformer for files with a .mjs extension.

Step 4: Update Babel Configuration

If you have a .babelrc file or a babel.config.js file in your project, make sure it includes the necessary plugins and presets to handle ECMAScript modules. For example:

{
  "presets": [
    [
      "@babel/preset-env",
      {
        "targets": {
          "node": "current"
        }
      }
    ]
  ]
}

This configuration ensures that Babel transpiles the ECMAScript modules to a compatible format that can be understood by Jest.

Step 5: Retry Running Jest

Try running Jest again with the --experimental-vm-modules flag to explicitly enable support for ECMAScript modules:

npx jest --experimental-vm-modules

By going through these additional steps, you should be able to resolve the “Cannot use import statement outside a module” error and successfully run your Jest tests with ECMAScript modules.