Why and What to Test

Why and What to Test

In this lesson — part of Testing Fundamentals — you'll learn why and what to test in JavaScript and why it matters in real work.

Why it matters

Tests prove your code works and keep it working as you change it.

Key ideas

• What to test
• Arrange-Act-Assert
• Running a test suite
• Good vs. brittle tests

In practice

Here's how it looks in idiomatic JavaScript:

// example.test.js (run with: node --test)
import { test } from "node:test";
import assert from "node:assert/strict";

test("adds numbers", () => {
  assert.equal(2 + 3, 5);
});

JavaScript note: Node's built-in test runner (node:test, run via node --test) needs no dependencies, while Jest and Vitest are the popular third-party choices.

Try it yourself

Exercise: In JavaScript, write three tests for a function that reverses a string.

Recap

You now understand why and what to test and can apply it in JavaScript. Mark this lesson complete and continue to the next one.