Why Testing Matters

Tests catch bugs early and provide confidence when refactoring. Investment in testing saves debugging time later.

Getting Started

npm install --save-dev jest

# In package.json
"scripts": {
  "test": "jest",
  "test:watch": "jest --watch"
}

Basic Tests

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

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

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

describe('add function', () => {
  it('handles negative numbers', () => {
    expect(add(-1, -2)).toBe(-3);
  });
});

Mocking

const fetchData = jest.fn().mockResolvedValue({ data: 'test' });

test('fetches data', async () => {
  const result = await fetchData();
  expect(result.data).toBe('test');
  expect(fetchData).toHaveBeenCalled();
});

Conclusion

Start with simple unit tests and expand coverage gradually. Even 50% coverage is better than none.