-code With Mosh- | Mastering Javascript Unit Testing

function applyDiscount(user, total) { if (user.type === 'VIP') return total * 0.8; return total; }

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. -Code With Mosh- Mastering JavaScript Unit Testing

npm run test:coverage A terminal window filled with green dots. Then, he did something reckless. function applyDiscount(user, total) { if (user

test('should use credit card if PayPal fails', async () => { // Mock the failing PayPal service const mockPayPal = jest.fn().mockRejectedValue(new Error('Timeout')); const result = await processPayment(mockPayPal, 'creditCard'); const total = 100

Because Leo finally understood: writing tests wasn't about proving his code worked today. It was about having the courage to change it tomorrow.