Testing Basics

Jest · Reference cheat sheet

📋 Overview

Jest is a zero/low-config JS test runner with built-in matchers, mocks, and coverage. Files: *.test.js|ts / *.spec.js|ts. Structure with describe / test (it).

🔧 Core concepts

💡 Examples

Minimal:

function add(a, b) {
  return a + b;
}

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

Group:

describe('cart', () => {
  test('starts empty', () => {
    expect([]).toHaveLength(0);
  });
});

Run:

npx jest
npx jest auth.test.ts -t "login"
npx jest --coverage

⚠️ Pitfalls

• toBe is reference equality — use toEqual for objects.
• Forgotten await passes false greens.
• Snapshot spam — prefer explicit field asserts for APIs.

🔗 Related

• Matchers
• Async
• API testing
• Auth testing