What is the difference between unit, integration, and end-to-end tests?

Answer

Unit tests verify a single function or class in isolation, often with dependencies mocked, and run very fast. Integration tests check that multiple components work correctly together—for example, code talking to a real database—covering the seams that unit tests miss. End-to-end (e2e) tests exercise the entire application from the user's perspective, such as driving a browser through a full workflow, giving the highest confidence but the slowest, most fragile feedback. A healthy suite uses many unit tests, fewer integration tests, and a small set of e2e tests.