jest

name	jest
description	Best practices for writing JavaScript and TypeScript tests with Jest. Use when asked to write unit tests, improve test coverage, set up mocking, test async code, or test React components with Testing Library. Covers test structure, mock strategies, snapshot testing, and common Jest matchers.

Jest Testing Best Practices

Test Structure

• Name test files with .test.ts or .test.js suffix
• Place test files next to the code they test or in a dedicated __tests__ directory
• Use descriptive test names that explain the expected behavior
• Use nested describe blocks to organize related tests
• Follow the pattern: describe('Component/Function/Class', () => { it('should do something', () => {}) })

Effective Mocking

• Mock external dependencies (APIs, databases, etc.) to isolate your tests
• Use jest.mock() for module-level mocks
• Use jest.spyOn() for specific function mocks
• Use mockImplementation() or mockReturnValue() to define mock behavior
• Reset mocks between tests with jest.resetAllMocks() in afterEach

Testing Async Code

• Always return promises or use async/await syntax in tests
• Use resolves/rejects matchers for promises
• Set appropriate timeouts for slow tests with jest.setTimeout()

Snapshot Testing

• Use snapshot tests for UI components or complex objects that change infrequently
• Keep snapshots small and focused
• Review snapshot changes carefully before committing

Testing React Components

• Use React Testing Library over Enzyme for testing components
• Test user behavior and component accessibility
• Query elements by accessibility roles, labels, or text content
• Use userEvent over fireEvent for more realistic user interactions