jest-testing-patterns

Unit tests in Jest use describe and it blocks to organize test suites, with assertions to verify function behavior. Jest provides built-in matchers and works with Node.js environments to validate logic before deployment.

Jest mocks replace real implementations with test doubles using jest.fn() for functions and jest.mock() for entire modules. Mocks let you control behavior, verify calls, and isolate code under test from dependencies.

Async testing in Jest uses async/await, Promises, or done callbacks in test functions. Jest waits for async operations to complete before validating results, supporting setTimeout, API calls, and other deferred logic.

Snapshot tests capture component or function output and compare future runs against saved snapshots. Jest detects unintended changes automatically, making them useful for UI components and complex data structures.

Jest supports TypeScript through proper configuration and tooling integration. The patterns for describe blocks, mocks, spies, and assertions work identically across JavaScript and TypeScript codebases.

Jest offers beforeEach, afterEach, beforeAll, and afterAll hooks to run code before or after tests. These hooks manage test isolation, reset state, and clean up resources across your test suite.