Mocks do not swallow the system under test

When a test mocks every export from the file it is supposed to test, the test exercises the mock — not the production code. The function under test is never called. The test will pass even if the real implementation is completely deleted. This pattern appears in AI-generated tests where the model confuses "mock the dependencies of X" with "mock X itself." The test suite accumulates green results that have zero correlation with production behavior, creating false confidence. ISO-25010:2011 testability requires that tests exercise the actual system under test.

Mock external dependencies (databases, APIs, third-party SDKs), never the function being tested. The unit under test must be imported and called directly:

// Before: tests the mock, not the real calculateTax
import { calculateTax } from './tax'
vi.mock('./tax', () => ({ calculateTax: vi.fn(() => 100) }))
it('calculates tax', () => {
  expect(calculateTax(1000)).toBe(100) // always true
})

// After: real function runs, only its dependency is mocked
import { calculateTax } from './tax'
import { taxRateApi } from './tax-rate-api'
vi.mock('./tax-rate-api', () => ({ taxRateApi: { fetch: vi.fn(() => 0.1) } }))
it('calculates tax at 10% rate', async () => {
  const tax = await calculateTax(1000)
  expect(tax).toBe(100)
})