Verify the Test Fails Before Writing Code

Always run your new test and watch it fail before writing implementation code. A test that passes immediately either tests nothing meaningful or the feature already exists.

Incorrect (never seeing red):

// Write test
test('validates email format', () => {
  expect(isValidEmail('user@example.com')).toBe(true)
})

// Immediately write implementation without running test
function isValidEmail(email: string): boolean {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)
}

// Run tests - passes, but did the test ever fail?
// Could be testing the wrong function or have a typo

Correct (verify RED before GREEN):

// Step 1: Write test
test('validates email format', () => {
  expect(isValidEmail('user@example.com')).toBe(true)
})

// Step 2: Run test - see it fail
// Error: isValidEmail is not defined
// This confirms the test is wired up correctly

// Step 3: Write minimal stub
function isValidEmail(email: string): boolean {
  return false
}

// Step 4: Run test - see it fail with correct assertion
// Expected: true, Received: false
// This confirms the assertion is testing the right thing

// Step 5: Implement
function isValidEmail(email: string): boolean {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)
}

// Step 6: Run test - see it pass (GREEN)

Why this matters:

• Confirms test is actually running
• Validates the assertion checks what you intend
• Catches copy-paste errors from other tests
• Proves the test can detect failure

Reference: Test Driven Development - Martin Fowler