Test-Driven Development

Iron Law

"NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST"

If code exists before its test, DELETE it and restart. No exceptions.

The Cycle

RED Phase

1. Write ONE minimal test demonstrating required behavior
2. Run test - confirm it FAILS (not errors)
3. Verify failure is for the RIGHT reason (missing feature, not syntax)

GREEN Phase

1. Write SIMPLEST code to pass the test
2. No feature creep, no over-engineering
3. Run test - confirm it PASSES
4. Confirm ALL other tests still pass

REFACTOR Phase (only after GREEN)

1. Eliminate duplication
2. Improve naming
3. Extract helpers if needed
4. Maintain test passage

Verification Checklist

• Every function has an associated test
• Observed each test fail BEFORE implementation
• Failures occurred for correct reasons
• Implemented minimal passing code
• All tests pass with clean output
• Edge cases and error conditions covered

Red Flags (STOP and restart)

• Code exists before tests
• Tests pass immediately upon writing
• "Just this once" rationalization
• "Tests can come later" thinking

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