Testing Introduction

Automated tests verify your code does what it should — and *keep* verifying it as the code changes. They're how you ship confidently, refactor without fear, and catch bugs before your users do.

Why test at all

• Catch bugs early — when they're cheap and quick to fix.
• Refactor safely — change internals knowing tests will flag any regression.
• Document behavior — tests show how code is meant to be used.
• Enable continuous delivery — automated checks let you deploy often, safely.

Bugs get costlier over time

The whole point of testing is to shift bug-catching left — toward the cheap end of that table.

What makes a good test

Good tests are fast (run in milliseconds), isolated (independent of each other), deterministic (same result every run), and readable (a clear statement of intended behavior). A slow, flaky, or cryptic test does more harm than good.

The landscape

Modern stacks use frameworks like Jest / Vitest (JS/TS), JUnit (Java), or pytest (Python). They all share the same core ideas this course covers: test types, writing tests well, test doubles, and TDD.