javascript-unit-testing

Unit tests for JavaScript use Jest's test framework with the AAA pattern: Arrange setup, Act on code, Assert results. Write small, focused tests that verify single behaviors, run with npm test or yarn test, and keep tests isolated and fast by mocking external dependencies.

Mock dependencies using stubs, mocks, and dependency injection to isolate the code under test. Jest provides built-in mocking utilities; inject dependencies as parameters or use constructor injection so tests control what the code receives, avoiding real API calls or database hits.

Async testing in Jest uses async/await, Promises, or callbacks with done() parameter. For timers, use fake timers with jest.useFakeTimers() to control time advancement, then jest.runAllTimers() or jest.advanceTimersByTime() to trigger async behavior without delays in tests.

Yes, Jest testing patterns work directly with TypeScript projects. The same AAA structure, mocking strategies, and async techniques apply; Jest handles TypeScript compilation automatically with proper configuration, so you write type-safe tests with the same maintainability benefits.

Flaky tests often stem from incomplete mocking, untamed async code, or testing implementation details instead of behavior. Ensure all external dependencies are mocked, async operations complete before assertions, and tests verify outcomes users care about, not internal state or timing-dependent behavior.

Refactor tests by extracting setup into helper functions, naming tests clearly with intent, using consistent patterns like AAA across the suite, and removing redundant assertions. Group related tests, isolate each test fully, and keep the test readable so intent is obvious without comments.