Jest is a test library for JavaScript developed by Facebook (https://jestjs.io/). Jest is widely used with React and provides lots of useful features for testing. You can create a snapshot test, where you can take snapshots from React trees and investigate how states are changing. Jest also has mock functionalities that you can use to test, for example, your asynchronous REST API calls. Jest also provides functions that are required for the assertions in your test cases.

We will first see how you can create a simple test case for a basic JavaScript function that performs some simple calculations. The following function takes two numbers as arguments and returns the product of the numbers:

// multi.js
export const calcMulti = (x, y) => {
    x * y;
}

The following code shows a Jest test for the preceding function. The test case starts with a test method that runs the...