Code That Checks Code

Unit tests automatically verify that individual small functions (units) work correctly. They are your project's safety net.

Why Write Tests?

• Catch regressions: Change code in one place, tests instantly tell you if something broke elsewhere.
• Confidence to refactor: Restructure code fearlessly, knowing tests will catch any mistakes.
• Living documentation: Tests show exactly how a function is supposed to behave.

Popular Tools

• Jest: Industry standard from Meta. Widely used in React projects.
• Vitest: Modern, ultra-fast alternative (built for Vite projects).

// sum.js
export function sum(a, b) { return a + b; }

// sum.test.js (Vitest)
import { test, expect } from 'vitest';
import { sum } from './sum.js';

test('adds two positive numbers', () => {
  expect(sum(2, 3)).toBe(5);
});

test('handles negative numbers', () => {
  expect(sum(-1, 1)).toBe(0);
});

test('handles zero', () => {
  expect(sum(0, 0)).toBe(0);
});