Unit tests for logic
🎯You'll be able to
Write unit tests for logic such as money formatting, including edge cases.
🏠The everyday picture
Before a big wedding feast, the cook tastes the masala on its own. If the spice blend is wrong, it's easier to catch now than after it's in fifty dishes.
📘What it really is
- Keep tricky logic in plain functions, separate from components, so it's easy to test:
cartTotal(items, offers, deliveryFee). - A unit test with Vitest or Jest:
import { cartTotalPaise } from './cart';
test('applies free delivery at ₹499 and above', () => {
const items = [{ pricePaise: 25000, qty: 2 }]; // ₹500
expect(cartTotalPaise(items, { deliveryPaise: 3000 })).toBe(50000);
});
- Each test has three parts: arrange (set up data), act (call the function), assert (check the result).
- Cover edge cases: empty cart, exactly on a threshold, just below it, large quantities, rounding.
- Use table-driven tests (
test.each) for many similar cases. - Work in paise (whole numbers) to avoid floating-point surprises (
0.1 + 0.2isn't exactly0.3). - When a bug is reported, first write a test that reproduces it, then fix the code so the test passes.
📝A worked example
Kirana's free-delivery rule: "Free delivery on orders of ₹499 or more; otherwise ₹30."
| Cart value | Expected total | First result |
|---|---|---|
| ₹0 (empty) | ₹0 (no delivery fee on empty cart) | ₹30 ✗ |
| ₹498.99 | ₹528.99 | ✓ |
| ₹499.00 | ₹499.00 | ₹529.00 ✗ (used > instead of >=) |
| ₹1,200 | ₹1,200 | ✓ |
Two bugs were found by edge-case tests before any customer saw them.
🌍Where it's used
- Prices, taxes, discounts and fees.
- Validation: pin codes, phone numbers, dates.
- Formatting: rupees, dates in IST, plurals.
📝Case scenario
A customer reports being charged a delivery fee on a ₹499 order, though the banner says free delivery from ₹499. You write a test for
exactly ₹499, see it fail, change > to >=, and keep the test so the bug can't quietly return.
🖱️Try it
Move your Kirana cart calculation into a plain function. Write at least eight tests covering empty cart, thresholds, offers and large
quantities, using test.each for similar cases.
⚠️Common mistakes
- Testing only the happy path. Sign: zero, negative or large amounts break in production. Fix: add edge cases.
- Using floats for money. Sign: ₹0.1 + ₹0.2 isn't ₹0.3. Fix: store paise as whole numbers and format at the end.
🛠️For your project
lib/money.ts in the starter is broken on purpose; tests/unit/money.test.ts has 8 tests that pass once you fix it.
✅Quick check
What are the three parts of a unit test?
Show the answer
Arrange the data, act by calling the code, assert the result.
What should you do first when a bug is reported?
Show the answer
Write a test that reproduces it, then fix the code until the test passes.