🧪 Testing JavaScript with Jest

Jest is a modern testing framework by Facebook that's great for testing JavaScript applications. It supports assertions, mocking, snapshot testing, and more.

📦 Step 1: Install Jest

# Initialize project
npm init -y

# Install Jest
npm install --save-dev jest

Add this to your package.json to run tests using the npm test command:

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

🧮 Step 2: Write a Function to Test

// math.js
function add(a, b) {
  return a + b;
}
module.exports = add;

🧪 Step 3: Create a Test File

// math.test.js
const add = require('./math');

test('adds 2 + 3 to equal 5', () => {
  expect(add(2, 3)).toBe(5);
});

🚀 Step 4: Run the Tests

npm test

You’ll see a green success message if everything is correct ✅

🔍 Common Matchers

• .toBe(value) – Exact match
• .toEqual(object) – For objects/arrays
• .toContain(item) – Check if array includes value
• .toBeTruthy(), .toBeFalsy() – Booleans
• .toThrow() – Expect error to be thrown

🧠 Mocking Example

Simulate a function’s behavior without calling the real implementation:

const fetchData = jest.fn(() => 'mocked data');
test('fetchData returns mocked data', () => {
  expect(fetchData()).toBe('mocked data');
});