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)
})

• ID: mocks-do-not-swallow-system-under-test

• Severity: high

• What to look for: Walk all test files. For each test file, identify the file under test (typically the file with the same basename minus .test/.spec). Count all vi.mock(/jest.mock(/sinon.stub(/td.replace( calls in the test file. If the mock target IS the file under test (e.g., import { foo } from './my-module' is followed by vi.mock('./my-module') and the test then calls the mocked foo), the test is testing the mock — not the real code. Count test files where 100% of imports from the file under test are mocked.

• Pass criteria: 0 test files mock 100% of the system under test. Report: "X test files inspected, Y with reasonable mocking, 0 mock-saturated."

• Fail criteria: At least 1 test file mocks every export from the file it's supposedly testing.

• Skip (N/A) when: Project has 0 test files OR no mock library is in use.

• Do NOT pass when: A test file mocks every export from the file under test even if it then calls "the real function" — once the module is mocked, all subsequent imports get the mock.

• Cross-reference: For multi-test-framework drift detection, the Multi-Session Drift audit (ai-slop-code-drift) catches projects using both Jest and Vitest in parallel.

• Detail on fail: "1 mock-saturated test: tests/users.test.ts mocks './users' completely — every function it calls is a mock, the real code is never executed"

• Remediation: When the test mocks the system under test, it's testing the mock, not the code. Mock external dependencies (databases, APIs, third parties), not the unit being tested:

  // Bad: mocks the function being tested
  import { calculateTax } from './tax'
  vi.mock('./tax', () => ({
    calculateTax: vi.fn(() => 100),
  }))
  it('calculates tax', () => {
    expect(calculateTax(1000)).toBe(100)  // tests the mock, not the real fn
  })
  
  // Good: mock dependencies, test the real function
  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)  // real function, mocked dependency
    expect(tax).toBe(100)
  })