How should I balance unit, integration, and E2E tests?
The classic guidance is many unit tests, fewer integration tests, and few E2E tests. Unit tests are cheap and fast; E2E tests are expensive and slow. A modern view puts more weight on integration tests, which give high confidence at reasonable cost.
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Unit tests test a single component or function in isolation. Integration tests test multiple units working together. End-to-end tests test the full app through a real browser, simulating real user journeys like login and checkout.
A guidance that says have many unit tests at the base, fewer integration tests in the middle, and few end-to-end tests at the top. The shape reflects cost and speed: unit tests are cheap and fast, E2E tests are expensive and slow.
Critical user journeys like signup, login, checkout, and the main flow of your app. E2E tests are slow and expensive, so reserve them for the paths that matter most, not for every component.
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