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.
Last Tuesday was the breaking point. A simple pull request to update a discount function caused a catastrophic cascade. The login failed. The cart emptied. The CEO’s test account showed a total price of . The company had to pay customers to buy things.
"So," she said. "Did Mosh save you?"
function applyDiscount(user, total) { if (user.type === 'VIP') return total * 0.8; return total; }
"Don't test the implementation. Test the behavior. If you're afraid to change your code, your tests are bad." -Code With Mosh- Mastering JavaScript Unit Testing
Leo had been a JavaScript developer for three years. He could spin up a React component in his sleep and chain promises like a poet. Yet, every Friday evening, the same dread washed over him as he typed npm run build .
FAIL checkout.test.js ✕ calculateTax should add 8% sales tax (5ms) ✕ applyDiscount should not apply to non-VIP (2ms) The tests screamed instantly. The broken line was caught before it ever reached production. test('apply 20% discount to VIP users', () =>
He felt a strange rush. It wasn't the dopamine hit of shipping messy code fast. It was the quiet confidence of building a brick wall, one perfect brick at a time. The hardest chapter was Mocks & Stubs . Leo had an API call to fetchUserPaymentMethod . In production, this called a slow database. In tests, it was a nightmare.