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

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)

javascript-typescript-jest