Getting Started

Install Jest using npm:

npm install --save-dev jest

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 npm 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.

Additional Configuration #

Using Babel #

To use Babel, install the babel-jest and babel-polyfill packages:

npm install --save-dev babel-jest babel-polyfill

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-es2015 and babel-preset-react presets:

{
  "presets": ["es2015", "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.

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, install the ts-jest package:

npm install --save-dev test-jest

then modify your package.json so the jest section looks something like:

{
  "jest": {
    "transform": {
      ".(ts|tsx)": "<rootDir>/node_modules/ts-jest/preprocessor.js"
    },
    "testRegex": "(/__tests__/.*|\\.(test|spec))\\.(ts|tsx|js)$",
    "moduleFileExtensions": [
      "ts",
      "tsx",
      "js"
    ]
  }
}