Jest
🃏 Painless JavaScript Testing
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👩🏻💻 Easy Setup: Jest is a complete and easy to set up JavaScript testing solution. In fact, Jest works out of the box for any React project.
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🏃🏽 Instant Feedback: Failed tests run first. Fast interactive mode can switch between running all tests or only test files related to changed files.
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📸 Snapshot Testing: Jest can capture snapshots of React trees or other serializable values to simplify UI testing.
Getting Started
<generated_getting_started_start />
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.
Running from command line
You can run Jest directly from the CLI (if it's globally available in your PATH
, e.g. by npm install -g 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
To use Babel, install the babel-jest
and regenerator-runtime
packages:
npm install --save-dev babel-jest regenerator-runtime
Note: Explicitly installing regenerator-runtime
is not needed if you use npm
3 or 4 or Yarn
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 defineNODE_ENV
astest
. It will not usedevelopment
section like Babel does by default when noNODE_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 and the types for Jest.
npm install --save-dev ts-jest @types/jest
then modify your package.json
so the jest
section looks something like:
{
"jest": {
"transform": {
"^.+\\.tsx?$": "<rootDir>/node_modules/ts-jest/preprocessor.js"
},
"testRegex": "(/__tests__/.*|\\.(test|spec))\\.(ts|tsx|js)$",
"moduleFileExtensions": [
"ts",
"tsx",
"js",
"json"
]
}
}
<generated_getting_started_end />
Documentation
Learn more about using Jest at http://facebook.github.io/jest
Contributing
Send issues and pull requests with your ideas. For more information about contributing PRs and issues, see our Contribution Guidelines.
Good First Task is a great starting point for PRs.