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Unit vs integration vs end-to-end tests — what’s the difference?

Tags

#unit#integration#e2e

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Answer

Unit tests verify small pieces in isolation, integration tests verify collaboration between components, and end-to-end tests validate full user flows across the system.

Advanced answer

Deep dive

Each level trades speed for realism:

Unit: fast, deterministic, good for logic and edge cases.
Integration: medium speed, catches wiring/config issues.
E2E: slowest, highest confidence for critical flows.

A healthy suite balances all three with a clear goal per layer.

Examples

Checkout flow:

Unit: price calculation
Integration: service + DB
E2E: user adds to cart -> pays -> receipt

Common pitfalls

Too many E2E tests causing slow feedback.
Unit tests that mock everything and miss integration bugs.
Not running E2E in stable environments.

Interview follow-ups

How do you decide what belongs at each level?
How many E2E tests are enough?
How do you keep tests reliable in CI?

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#architecture#ddd#integration