Testing with Jest

Testing ensures your code works as expected and prevents bugs from reappearing (regressions).Jest is a delightful JavaScript Testing Framework with a focus on simplicity.

1. Getting Started

npm install jest --save-dev

Add to package.json:

"scripts": {
  "test": "jest"
}

2. Writing Your First Test

Create a file named math.test.js. Jest will automatically find any file ending in .test.js or .spec.js.

const { add } = require('./math');

test('adds 1 + 2 to equal 3', () => {
  expect(add(1, 2)).toBe(3);
});

3. Testing Asynchronous Code

Jest makes testing async functions easy with async/await support.

test('fetches user data', async () => {
  const user = await getUser(1);
  expect(user.name).toBe('John');
});

4. Mocking

Mocking allows you to isolate the code you're testing by replacing dependencies (like database calls) with fake implementations.

const axios = require('axios');
jest.mock('axios');

test('should fetch users', async () => {
  const users = [{ name: 'Bob' }];
  axios.get.mockResolvedValue({ data: users });

  const result = await fetchUsers();
  expect(result).toEqual(users);
});