TestingInterview questions and answers

Jest is the most popular JavaScript test runner — comes bundled with an assertion library (expect), mocking, snapshot testing, parallel execution, and watch mode. Created by Facebook for React but used across the JS ecosystem. Zero-config for most projects.

React Testing Library is a testing tool that encourages tests mimicking real user behavior — query by accessible roles, labels, and text content rather than internal class names or component state. Tests stay green through refactors that don't change behavior.

Unit: tests one function or module in isolation. Integration: tests several units working together (often with a real DB). E2E: tests the full application through its real UI and network calls. Each level finds different bugs; a healthy suite has all three in different ratios.

A classic guide: many fast unit tests at the base, fewer integration tests in the middle, very few slow E2E tests at the top. Optimizes for speed, stability, and maintainability. The pyramid keeps test runs short enough to actually run on every change.

Generic term for any stand-in for a real dependency in tests — includes fake, mock, stub, and spy. Test doubles let you isolate the code under test from slow, flaky, or hard-to-control collaborators like databases, APIs, and the clock.