Testing

Unit, integration, end-to-end, and load tests - plus TDD/BDD - that actually catch the bug.

Basic

Why Test At All? Tests are a safety net that let you change code without fear - this guide gives you the mental model that makes testing feel like a gift instead of a chore, before you write a single test.

Your First Unit Test Write and run a real unit test from scratch: learn the Arrange-Act-Assert shape, watch a test pass green and fail red, and pick up the habits that make a test worth keeping.

Intermediate

Unit, Integration & E2E Tests, Explained What the three levels of testing actually are, what each one catches and costs, and how to get the mix right so your suite stays fast and your failures point somewhere useful.

Mocking, Stubbing & Test Doubles What test doubles actually are, the difference between a dummy, stub, fake, spy, and mock, and the judgment call of when to fake a dependency versus use the real thing.

TDD & BDD, No-Nonsense What test-driven and behavior-driven development actually are, the red-green-refactor and Given/When/Then cycles, and a clear-eyed take on when each one earns its keep - and when it's just ritual.

Testing in CI (What Runs on Every Push) CI is a server that automatically runs your test suite on every push and pull request, so broken code can't quietly reach main - here's the mental model, what a run actually does, and how to keep it trustworthy.

Flaky Tests Why a test passes and fails with no code change, the usual culprits, and how to kill flakiness for good instead of hiding it behind retries.

Advanced

Load & Performance Testing Your tests prove the app is correct; load tests prove it survives a thousand people showing up at once. Here's the mental model, the metrics that actually matter (percentiles, not averages), and how to run one test and read where it breaks.