Jest

Jest is a JavaScript testing framework maintained by Meta (Facebook). It’s widely used for testing React applications, but it works with any JavaScript/TypeScript project.

It provides features like:

• Unit testing: testing small pieces of code (functions, components).
• Snapshot testing: ensuring UI doesn’t change unexpectedly.
• Mocking: simulating functions, APIs, or modules.
• Zero configuration: works out of the box for most setups.

Example: Testing a simple function

Let’s say you have a function sum.js:

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

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

Now, write a test for it in sum.test.js:

// sum.test.js
const sum = require('./sum');

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

// sum.test.js
const sum = require('./sum');

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

Running the test

If Jest is installed (npm install --save-dev jest), add this to package.json:

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

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

Then run:

npm test

npm test

✅ Expected output:

PASS  ./sum.test.js
✓ adds 1 + 2 to equal 3 (5 ms)

PASS  ./sum.test.js
✓ adds 1 + 2 to equal 3 (5 ms)