🧪 Jest Testing Basics

Jest is a zero-config testing framework for JavaScript and React.

Your First Test

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

Test Structure

describe('Calculator', () => {
  test('adds numbers', () => {
    expect(add(1, 2)).toBe(3);
  });

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

Common Matchers

expect(value).toBe(5);              // Exact match
expect(value).toEqual({ a: 1 });    // Deep equality
expect(value).toBeTruthy();         // Truthy
expect(value).toContain(2);         // Array contains
expect(text).toMatch(/jest/i);      // Regex match

Running Jest

npm test                    # Run all tests
npm test -- --coverage      # With coverage
npm test -- --watch         # Watch mode

✅ Key Takeaways

• Jest is **zero-config** and works out of the box

• Use **test()** to write test cases

• Use **expect()** with **matchers** for assertions

• **describe()** groups related tests