babel-jest

What is babel-jest?

The babel-jest package is a Jest plugin that makes it easy to use Babel to transform your JavaScript tests. It automatically hooks Babel into Jest and compiles files before running them, allowing you to use the latest JavaScript features and syntax in your test files.

What are babel-jest's main functionalities?

Transforming test files with Babel

This configuration in Jest's setup file tells Jest to use babel-jest for transforming any files that match the regex pattern (in this case, any JavaScript files) before running the tests.

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

Using modern JavaScript syntax in tests

With babel-jest, you can write tests using modern JavaScript features like async/await without worrying about compatibility issues.

test('async/await works', async () => {
  const data = await fetchData();
  expect(data).toBe('expected data');
});

Custom Babel configuration

babel-jest allows you to specify a custom Babel configuration to tailor the transformation process to your project's needs.

{
  'presets': [['@babel/preset-env', { 'targets': { 'node': 'current' } }]],
  'plugins': ['@babel/plugin-proposal-class-properties']
}

Other packages similar to babel-jest

ts-jest

ts-jest is a TypeScript preprocessor with source map support for Jest. It allows you to use Jest to test projects written in TypeScript. It is similar to babel-jest but specifically tailored for TypeScript rather than general Babel transformations.

jest-transform-stub

jest-transform-stub is a package that allows you to stub out resources, like images or styles, during testing with Jest. It is similar to babel-jest in that it transforms assets, but it focuses on non-JavaScript assets instead of JavaScript code.

jest-vue-preprocessor

jest-vue-preprocessor is a preprocessor for Jest to handle .vue files, allowing you to test Vue components. It is similar to babel-jest in the sense that it preprocesses files for testing, but it is specifically designed for Vue components.

README

babel-jest

Babel jest plugin

Usage

If you are already using jest-cli, add babel-jest and it will automatically compile JavaScript code using Babel.

yarn add --dev babel-jest @babel/core

If you would like to write your own preprocessor, uninstall and delete babel-jest and set the config.transform option to your preprocessor.

Setup

Note: this step is only required if you are using babel-jest with additional code preprocessors.

To explicitly define babel-jest as a transformer for your JavaScript code, map .js files to the babel-jest module. Typescript files are also supported.

By default, it loads your existing Babel configuration (if any)

"transform": {
  "\\.[jt]sx?$": "babel-jest"
},

You can also pass further babel options

"transform": {
  "\\.[jt]sx?$": ["babel-jest", { "extends": "./babel.config.js", "plugins": ["babel-plugin-transform-import-meta"] }]
},

By default, babel-jest includes babel-preset-jest. In addition to the babel options, we introduce a new option, excludeJestPreset, which allows you to disable this behavior. Note that this will break jest.mock hoisting.

"transform": {
  "\\.[jt]sx?$": ["babel-jest", { "excludeJestPreset": true }],
}