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)