Jest is a testing framework maintained by Facebook to test React apps.

Install and Configure

$ yarn add —dev jest

If you started your app with create-react-app boilerplate a lot of the configuration will already exist. To start testing all you need to do is run npm test.

The file that you are testing should reside in a file that has the same name with .test.js ending. This file should reside in mirror directory structure vis a vi it’s counterpart.

Import the file at the top of the page. Typically you would create a wrapper 'describe' function for the main piece of code being tested with nested 'describe' statements for each smaller sub feature. Each feature is tested with a 'it' function, the first parameter is a string describing this feature and giving it a title. The next parameter is a function holding the main test logic. Each assertion is tested with expect(some code here).toEqual(what it should equal)

Here's an example.

                describe('Name of what's being tested, () => {

                      describe('name of function tested', () => {

                          it('description of path being tested', () => {

                              /* code logic here */

                                  /* some function input */
                                  /* expected function output */

Learn more at the official Jest tutorial, link below.