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)`