GettingStarted.md

id	title
getting-started	Getting Started

Install Jest using yarn:

yarn add --dev jest

Or npm:

npm install --save-dev jest

Note: Jest documentation uses yarn commands, but npm will also work. You can compare yarn and npm commands in the yarn docs, here.

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 or npm run 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.

If you prefer to import functions instead of using them globally, see ES6 Import.

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 or npm install jest --global) with a 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

Generate a basic configuration file

Based on your project, Jest will ask you a few questions and will create a basic configuration file with a short description for each option:

jest --init

Using Babel

To use Babel, install the babel-jest and @babel/core packages via yarn:

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

Don't forget to add a babel.config.js 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:

module.exports = {
  presets: ['@babel/preset-env', '@babel/preset-react'],
};

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

{
  "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

Jest supports TypeScript out of the box, via Babel.

However, there are some caveats to using Typescript with Babel, see http://artsy.github.io/blog/2017/11/27/Babel-7-and-TypeScript/. Another caveat is that Jest will not typecheck your tests. If you want that, you can use ts-jest.