🃏 Delightful JavaScript Testing

• 👩🏻‍💻 Developer Ready: Complete and ready 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.

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.

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.

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": {}
  }
}

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.

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

Learn more about using Jest on the official site!

• Getting Started
• Guides
• API Reference
• Configuring Jest

Show the world you're using Jest →

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The main purpose of this repository is to continue to evolve Jest, making it faster and easier to use. Development of Jest happens in the open on GitHub, and we are grateful to the community for contributing bugfixes and improvements. Read below to learn how you can take part in improving Jest.

Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.

Read our contributing guide to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to Jest.

To help you get your feet wet and get you familiar with our contribution process, we have a list of good first issues that contain bugs which have a relatively limited scope. This is a great place to get started.

Jest is MIT licensed.