Javascript-Typescript-Jest

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)`