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 TypeScript 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 TypeScript:

import { describe, it, expect } from "vitest";

function add(a: number, b: number) { return a + b; }

describe("add", () => {
  it("sums two numbers", () => {
    expect(add(2, 3)).toBe(5);
  });
});

TypeScript note: Vitest (and Jest) run TypeScript tests directly and infer expect matcher types, so a wrong-typed assertion fails at compile time too.

Try it yourself

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

Recap

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