jestjs/jest

👩🏻‍💻 Developer Ready: A comprehensive JavaScript testing solution. Works out of the box for most JavaScript projects.

🏃🏽 Instant Feedback: Fast, interactive watch mode only runs test files related to changed files.

📸 Snapshot Testing: Capture snapshots of large objects to simplify testing and to analyze how they change over time.

See more on jestjs.io

Install Jest using yarn:

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:

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

Add the following section to your package.json:

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

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.