Jest

🃏 Delightful JavaScript Testing

• 👩🏻‍💻 Easy Setup: Complete and easy to set-up JavaScript testing solution. Works out of the box for any React project.

• 🏃🏽 Instant Feedback: Fast interactive watch mode runs only test files related to changed files and is optimized to give signal quickly.

• 📸 Snapshot Testing: Capture snapshots of React trees or other serializable values to simplify testing and to analyze how state changes over time.

Getting Started

Install Jest using yarn:

yarn add --dev jest

Or via npm:

npm install --save-dev jest

The minimum supported Node version is v6.0.0 by default. If you need to support Node 4, refer to the Compatibility issues section.

Let's get started by writing a test for a hypothetical function that adds two numbers. First, create a sum.js file:

function sum(a, b) {
  return a + b;
}
module.exports = sum;

Then, create a file named sum.test.js. This will contain our actual test:

const sum = require('./sum');

test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3);
});

Add the following section to your package.json:

{
  "scripts": {
    "test": "jest"
  }
}

Finally, run yarn test and Jest will print this message:

PASS  ./sum.test.js
✓ adds 1 + 2 to equal 3 (5ms)

You just successfully wrote your first test using Jest!

This test used expect and toBe to test that two values were exactly identical. To learn about the other things that Jest can test, see Using Matchers.

Running from command line

You can run Jest directly from the CLI (if it's globally available in your PATH, e.g. by yarn global add jest) with variety of useful options.

Here's how to run Jest on files matching my-test, using config.json as a configuration file and display a native OS notification after the run:

jest my-test --notify --config=config.json

If you'd like to learn more about running jest through the command line, take a look at the Jest CLI Options page.

Additional Configuration

Using Babel

Babel is automatically handled by Jest using babel-jest. You don't need install anything extra for using Babel.

Note: If you are using a babel version 7 then you need to install babel-core@^7.0.0-0 and @babel/core with the following command:

yarn add --dev 'babel-core@^7.0.0-0' @babel/core

Don't forget to add a .babelrc file in your project's root folder. For example, if you are using ES6 and React.js with the babel-preset-env and babel-preset-react presets:

{
  "presets": ["env", "react"]
}

You are now set up to use all ES6 features and React specific syntax.

Note: If you are using a more complicated Babel configuration, using Babel's env option, keep in mind that Jest will automatically define NODE_ENV as test. It will not use development section like Babel does by default when no NODE_ENV is set.

Note: If you've turned off transpilation of ES modules with the option { "modules": false }, you have to make sure to turn this on in your test environment.

{
  "presets": [["env", {"modules": false}], "react"],
  "env": {
    "test": {
      "presets": [["env"], "react"]
    }
  }
}

Note: babel-jest is automatically installed when installing Jest and will automatically transform files if a babel configuration exists in your project. To avoid this behavior, you can explicitly reset the transform configuration option:

// package.json
{
  "jest": {
    "transform": {}
  }
}

Using webpack

Jest can be used in projects that use webpack to manage assets, styles, and compilation. webpack does offer some unique challenges over other tools. Refer to the webpack guide to get started.

Using TypeScript

To use TypeScript in your tests you can use ts-jest.

Documentation

Learn more about using Jest on the official site!

• Getting Started
• Guides
• API Reference
• Configuring Jest

Badge

Show the world you're using Jest →

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Contributing

Send issues and pull requests with your ideas. For more information about contributing PRs and issues, see our Contribution Guidelines.

Good First Issue is a great starting point for PRs.