Using Jest

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

To demonstrate the syntax, we will see how to create a test case for a basic TypeScript function that performs some simple calculations. The following function takes two numbers as arguments and returns the product of the numbers:

// multi.ts
export const calcMulti = (x: number, y: number): number => {
  return x * y;
}

The following code snippet shows a Jest test for the preceding function:

// multi.test.ts
import { calcMulti } from './multi';
test("2 * 3...