-code With Mosh- Mastering Javascript Unit Testing Apr 2026

He opened checkout.js and deliberately deleted a single line—the tax calculation.

"So," she said. "Did Mosh save you?"

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 .

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.

"You write the test first ," Mosh explained. "You watch it fail. Then you write just enough code to pass. This forces you to ask: What do I actually need? "

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'); He opened checkout

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.

"That’s it," Sarah said, her voice eerily calm. "You’re not writing a single line of new code until you learn how to test the old code."

Leo decided to rewrite the cursed discount function. He opened a new file: discount.test.js . He could spin up a React component in

He typed:

Leo would sigh, dig through 2,000 lines of spaghetti logic, find the bug, fix it, and pray he hadn’t broken something else. He was a firefighter, not an engineer. His code worked—until it didn't.

His boss, Sarah, would inevitably Slack him: “Hey Leo, the checkout button broke again. Also, the user profile picture is showing up on the invoice page.”

He ran the tests again.