How to test a React App, with Jest and react-testing-library

Prerequisites: An existing React app.

You can find the full source code @ github: https://github.com/alexadam/project-templates/tree/master/projects/react-app-tests

Setup

Install Jest and react-testing-library

yarn add --dev jest @types/jest ts-jest @testing-library/react @testing-library/jest-dom

In the project's root folder, create a file named jest.config.js and add:

 react-app/jest.config.js
module.exports = {
  roots: ["./src"],
  transform: {
    "^.+\\.tsx?$": "ts-jest"
  },
  testEnvironment: 'jest-environment-jsdom',
  setupFilesAfterEnv: [
    "@testing-library/jest-dom/extend-expect"
  ],
  testRegex: "^.+\\.(test|spec)\\.tsx?$",
  moduleFileExtensions: ["ts", "tsx", "js", "jsx", "json", "node"]
};

Update package.json by adding a new script entry:

 react-app/package.json
"scripts": {
    ...
    "test": "jest",
    ...

Create a Test

We'll test the basic React app created from scratch here: https://alexadam.dev/blog/create-react-project.html

To test the Numbers component, create a file named numbers.test.tsx, in the src folder:

 react-app/src/numbers.test.tsx
import React, { Props } from "react";
import { fireEvent, render, screen } from "@testing-library/react";
import Numbers from "./numbers";

describe("Test <Numbers />", () => {

  test("Should display 42", async () => {
    render(<Numbers initValue={42} />);
    const text = screen.getByText("Number is 42");
    expect(text).toBeInTheDocument();
  });

  test("Should increment number", async () => {
    const { container }  = render(<Numbers initValue={42} />)

    const incButton =  screen.getByText('+')

    fireEvent(
      incButton,
      new MouseEvent('click', {
        bubbles: true,
        cancelable: true,
      }),
    )

    const text = screen.getByText("Number is 43");
    expect(text).toBeInTheDocument();
  });

  test("Should decrement number", async () => {
    const { container }  = render(<Numbers initValue={42} />)

    const decButton =  screen.getByText('-')

    fireEvent.click(decButton)

    const text = screen.getByText("Number is 41");
    expect(text).toBeInTheDocument();
  });

});

Run the tests:

yarn test

See the results:

$> yarn test
yarn run v1.22.17
$ jest
 PASS  src/numbers.test.tsx
  Test <Numbers />
    ✓ Should display 42 (29 ms)
    ✓ Should increment number (14 ms)
    ✓ Should decrement number (8 ms)

Test Suites: 1 passed, 1 total
Tests:       3 passed, 3 total
Snapshots:   0 total
Time:        2.893 s
Ran all test suites.
✨  Done in 3.88s.