Jest is a JavaScript testing framework designed to ensure correctness of any JavaScript codebase. It allows you to write tests with an approachable, familiar and feature-rich API that gives you results quickly.

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 …

DOM Testing If you'd like to assert, and manipulate your rendered components you can use @testing-library/react, Enzyme, or React's TestUtils. The following example use @testing-library/react. @testing-library/react

Jest is a universal testing platform, with the ability to adapt to any JavaScript library or framework. In this section, we'd like to link to community posts and articles about integrating Jest into popular JS libraries.

Mock functions allow you to test the links between code by erasing the actual implementation of a function, capturing calls to the function (and the parameters passed in those calls), capturing instances of constructor functions when instantiated with new, and allowing test-time configuration of return values.

Just like the describe and test blocks Jest calls the before* and after* hooks in the order of declaration. Note that the after* hooks of the enclosing scope are called first. For example, here is how you can set up and tear down resources which depend on each other:

When you're writing tests, you often need to check that values meet certain conditions. expect gives you access to a number of "matchers" that let you validate different things.

Another class of functions that is often considered difficult to test is code that directly manipulates the DOM. Let's see how we can test the following snippet of jQuery code that listens to a click event, fetches some data asynchronously and sets the content of a span.

The jest command line runner has a number of useful options. You can run jest --help to view all available options. Many of the options shown below can also be used together to run tests exactly the way you want. Every one of Jest's Configuration options can …

The native timer functions (i.e., setTimeout(), setInterval(), clearTimeout(), clearInterval()) are less than ideal for a testing environment since they depend on real time to elapse. Jest can swap out timers with functions that allow you to control the passage of time.