Good and Bad Tests

Good Tests

Integration-style: Test through real interfaces, not mocks of internal parts.

typescript

// GOOD: Tests observable behavior
test('user can checkout with valid cart', async () => {
  const cart = createCart()
  cart.add(product)
  const result = await checkout(cart, paymentMethod)
  expect(result.status).toBe('confirmed')
})

Characteristics:

Tests behavior users/callers care about
Uses public API only
Survives internal refactors
Describes WHAT, not HOW
One logical assertion per test

Bad Tests

Implementation-detail tests: Coupled to internal structure.

typescript

// BAD: Tests implementation details
test('checkout calls paymentService.process', async () => {
  const mockPayment = jest.mock(paymentService)
  await checkout(cart, payment)
  expect(mockPayment.process).toHaveBeenCalledWith(cart.total)
})

Red flags:

Mocking internal collaborators
Testing private methods
Asserting on call counts/order
Test breaks when refactoring without behavior change
Test name describes HOW not WHAT
Verifying through external means instead of interface

typescript

// BAD: Bypasses interface to verify
test('createUser saves to database', async () => {
  await createUser({name: 'Alice'})
  const row = await db.query('SELECT * FROM users WHERE name = ?', ['Alice'])
  expect(row).toBeDefined()
})

// GOOD: Verifies through interface
test('createUser makes user retrievable', async () => {
  const user = await createUser({name: 'Alice'})
  const retrieved = await getUser(user.id)
  expect(retrieved.name).toBe('Alice')
})