Test Behavior Not Implementation

Tests should verify what the code does (behavior), not how it does it (implementation). Implementation-coupled tests break during refactoring even when behavior is preserved.

Incorrect (testing implementation details):

test('sortUsers calls quicksort with correct comparator', () => {
  const quicksortSpy = jest.spyOn(sortUtils, 'quicksort')

  sortUsers(users, 'name')

  expect(quicksortSpy).toHaveBeenCalledWith(
    users,
    expect.any(Function)
  )
  // Breaks if we switch to mergesort, even though behavior is identical
})

test('caches user in internal Map', () => {
  const service = new UserService()
  service.getUser('123')

  // Testing private implementation detail
  expect(service['cache'].has('123')).toBe(true)
  // Breaks if we change cache structure
})

Correct (testing observable behavior):

test('sortUsers returns users ordered by name', () => {
  const users = [
    { name: 'Charlie', id: '1' },
    { name: 'Alice', id: '2' },
    { name: 'Bob', id: '3' }
  ]

  const sorted = sortUsers(users, 'name')

  expect(sorted.map(u => u.name)).toEqual(['Alice', 'Bob', 'Charlie'])
  // Passes regardless of sorting algorithm used
})

test('getUser returns same instance on repeated calls', () => {
  const service = new UserService()

  const first = await service.getUser('123')
  const second = await service.getUser('123')

  expect(first).toBe(second)
  // Tests caching behavior without knowing how it's implemented
})

Ask yourself:

• Would this test break if I refactored the internals?
• Am I testing public API or private implementation?
• Does this test document what users of this code care about?

Reference: Testing Implementation Details - Kent C. Dodds