javascript-typescript-jest

SKILL.md

Jest testing best practices for JavaScript and TypeScript projects with mocking, async handling, and React patterns.

Organize tests with descriptive names in nested describe blocks, using .test.ts/.test.js files placed alongside source code or in __tests__ directories

Mock external dependencies with jest.mock(), jest.spyOn(), and mockImplementation(), resetting between tests to prevent state leakage

Handle async code with promises, async/await, and resolves/rejects matchers; set timeouts for slow tests

Test React components with React Testing Library, querying by accessibility roles and labels, and using userEvent for realistic interactions

Includes 20+ common matchers covering truthiness, numbers, strings, arrays, objects, exceptions, and mock function assertions

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()