Testing

Writing automated code that verifies your app behaves correctly. Types: unit tests (individual functions), integration tests (systems working together), end-to-end tests (full user journeys).

Tests catch regressions, new code breaking old features. Without tests, every change is risky. With good test coverage, you can ship confidently and move fast without breaking things.

A developer writes a test that simulates a user completing checkout. When a later change accidentally breaks the payment flow, the test fails before it reaches production.