"What is the difference between unit testing and integration testing?"
This fundamental question checks whether you understand testing strategy and can make informed decisions about test coverage and architecture.
Define each type clearly with scope differences.
Explain when each is most valuable.
Discuss the testing pyramid concept.
Give a practical example from your experience.
Unit tests verify individual functions or methods in isolation, using mocks for dependencies. They run fast — thousands in seconds — and catch logic bugs early. Integration tests verify that multiple components work together correctly: API endpoints hitting real databases, service-to-service communication, or UI interacting with backend. They are slower but catch interface mismatches and configuration issues. I follow the testing pyramid: many unit tests at the base, fewer integration tests in the middle, and a small number of end-to-end tests at the top. In my last project, our 2,000 unit tests ran in 30 seconds while 200 integration tests took 8 minutes — both were essential but served different purposes.
Mention the testing pyramid to show strategic thinking.
Discuss trade-offs: speed vs confidence, isolation vs realism.
Reference specific frameworks you have used (Jest, pytest, JUnit).
Both are essential. Unit tests catch logic bugs cheaply and quickly. Integration tests catch wiring and configuration issues. A good strategy uses both in appropriate proportions.
E2E tests verify complete user workflows but are slow and brittle. Use them sparingly for critical paths. They sit at the top of the testing pyramid.
Generally no — the point of integration tests is to verify real interactions. Mock only external services you do not control (third-party APIs, payment gateways).
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