# Javascript Typescript Jest

> Best practices for writing JavaScript/TypeScript tests using Jest, including mocking strategies,…

Best practices for writing JavaScript/TypeScript tests using Jest, including mocking strategies, test structure, and common patterns.

- Page: https://ss33.ai/skills/github/javascript-typescript-jest-70829940765
- Publisher: github
- Category: [Software Development](https://ss33.ai/skills/category/software-development-t44z8z) › [Testing & QA](https://ss33.ai/skills/category/software-development-t44z8z/testing-and-qa-g56s6z)
- License: MIT
- Source: https://github.com/github/awesome-copilot/blob/cb64ad12e32737ae764becd6885462e9b4c7cb00/skills/javascript-typescript-jest/SKILL.md

## Skill

SKILL.md: https://ss33.ai/api/v1/ss33/download/github/javascript-typescript-jest-70829940765

### 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

## Common Jest Matchers
- Basic: `expect(value).toBe(expected)`, `expect(value).toEqual(expected)`
- Truthiness: `expect(value).toBeTruthy()`, `expect(value).toBeFalsy()`
- Numbers: `expect(value).toBeGreaterThan(3)`, `expect(value).toBeLessThanOrEqual(3)`
- Strings: `expect(value).toMatch(/pattern/)`, `expect(value).toContain('substring')`
- Arrays: `expect(array).toContain(item)`, `expect(array).toHaveLength(3)`
- Objects: `expect(object).toHaveProperty('key', value)`
- Exceptions: `expect(fn).toThrow()`, `expect(fn).toThrow(Error)`
- Mock functions: `expect(mockFn).toHaveBeenCalled()`, `expect(mockFn).toHaveBeenCalledWith(arg1, arg2)`