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Unit Testing

In short

You change one function at midnight before a deadline. How do you know you didn't break three others? Unit tests check each small piece on its own, in milliseconds.

Testing the smallest independent pieces of backend code — a single function or class — in isolation from the database, network, or other dependencies.

The blob character tests a single bulb in a tester beside a coiled string of lights.

Think of it like this

Testing each bulb first

Before stringing up Diwali lights, you test each bulb in a small tester. If the whole string fails later, you already know the bulbs are fine and the fault is elsewhere. A unit test checks one function by itself, with no database or network involved.

Watch it happen

Arrange, Act, Assert

Arrange

Act

Assert

1/6Arrange: build a cart with items worth 300 and 200 rupees, and a 10% coupon.

The big ideas

What you need to know

One behaviour per test

Each test checks one thing, like 'expired coupons are ignored'. When it fails, the name tells you what broke.

Fast and isolated

No database, files or network. Hundreds of unit tests should finish in seconds.

Pure functions are easiest

Functions that take inputs and return outputs, with no side effects, are a joy to test. Design for that.

Test the edges

Empty carts, zero, negative numbers, huge values. Bugs love the edges.

See it in code

Walk through the code

1import { test } from "node:test";2import assert from "node:assert/strict";3import { calculateTotal } from "./cart.js";4 5test("applies a 10% coupon", () => {6  const cart = [{ price: 300 }, { price: 200 }];7  const total = calculateTotal(cart, { percentOff: 10, expired: false });8  assert.equal(total, 450);9});10 11test("ignores an expired coupon", () => {12  const cart = [{ price: 300 }, { price: 200 }];13  assert.equal(calculateTotal(cart, { percentOff: 10, expired: true }), 500);14});

1/3Node's built-in test runner and strict assertions. No extra packages needed.

Run it with node --test. The same Arrange, Act, Assert shape works in Jest, Vitest or Mocha.

Save yourself the pain

Common mistakes

Oops

Testing private helper details, so every refactor breaks the tests.

Do this instead

Test what the function promises: inputs and outputs. Then you can rewrite its insides freely.

Oops

One giant test that checks ten different things.

Do this instead

Split it. Small, well-named tests tell you exactly what broke without reading the code.

Your mission

Test a pure function

~25 min

Quick check

Which of these is a unit test?

Go deeper

1Write unit tests for 3-4 pure utility functions in a backend project.
2Practice testing edge cases (empty input, invalid data), not just the happy path.
3Learn the AAA pattern: Arrange, Act, Assert.