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 …

Because TypeScript support in Babel is purely transpilation, Jest will not type-check your tests as they are run. If you want that, you can use ts-jest instead, or just run the TypeScript compiler tsc …

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.

In some scenarios, snapshot testing can potentially remove the need for unit testing for a particular set of functionalities (e.g. React components), but they can work together as well.

To test this function, we can use a mock function, and inspect the mock's state to ensure the callback is invoked as expected.

Since you are still testing promises, the test is still asynchronous. Hence, you will need to tell Jest to wait by returning the unwrapped assertion. Alternatively, you can use async/await in combination with …

It's common in JavaScript for code to run asynchronously. When you have code that runs asynchronously, Jest needs to know when the code it is testing has completed, before it can move …

The next time you run the tests, the rendered output will be compared to the previously created snapshot. The snapshot should be committed along with code changes. When a snapshot test fails, …

Jest uses "matchers" to let you test values in different ways. This document will introduce some commonly used matchers. For the full list, see the expect API doc.

Another test we might want to write for this module is one that asserts that the callback is called after 1 second. To do this, we're going to use Jest's timer control APIs to fast-forward time right in the …