After this lesson, you will be able to: Explain the testing pyramid, what each layer tests and what it costs, and why code coverage is a useful but misleading metric.
Testing is consistently undertaught and consistently asked about in interviews. This free intro frames the whole subtrack: the three layers of tests, how to balance them, and the honest truth about coverage numbers.
This is a free introductory lesson. No purchase required.
Anyone can write code that works once, on their machine, the day they wrote it. A professional writes code that keeps working after six months of other people changing things around it. Tests are how you get that confidence. They let you refactor without fear, catch regressions before users do, and prove your code does what you claim. This subtrack goes far deeper than the single Testing Fundamentals lesson in Engineering Fundamentals: a dedicated lesson per layer, per language, plus TDD and CI.
Three layers, from fast/cheap/many at the bottom to slow/expensive/few at the top. Unit tests: one function or module in isolation, run in milliseconds, you write hundreds. Integration tests: several pieces working together (an API route hitting a real database), run in seconds, you write dozens. End-to-end (e2e) tests: the whole app driven from the outside like a real user (a browser clicking buttons), run in tens of seconds, you write a handful for the critical journeys. The shape is a pyramid because each layer up costs more to write, run, and maintain, so you want most of your tests at the cheap, fast bottom.
Pick the best answer.