-code With Mosh- Mastering Javascript Unit Testing Apr 2026

test('calculate total price for two items', () => { // Arrange const cart = [{ price: 10 }, { price: 20 }]; // Act const result = calculateTotal(cart); // Assert expect(result).toBe(30); }); Leo typed along. For the first time, he ran npm test and saw that beautiful green checkmark. Passed.

Leo decided to rewrite the cursed discount function. He opened a new file: discount.test.js . -Code With Mosh- Mastering JavaScript Unit Testing

function applyDiscount(user, total) { if (user.type === 'VIP') return total * 0.8; return total; } test('calculate total price for two items', () =>

For the first time, Leo simulated a server crash on his laptop without breaking anything. He felt like a wizard. One week later, Leo walked into the sprint planning meeting. Sarah looked skeptical. Leo decided to rewrite the cursed discount function

It felt… clean. The next lesson hit him like a truck. Mosh introduced Test-Driven Development (TDD) .

test('apply 20% discount to VIP users', () => { const user = { type: 'VIP' }; const total = 100; const result = applyDiscount(user, total); expect(result).toBe(80); }); He ran it. The function didn't exist yet.