test-driven-development

Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor

graph LR
    red["RED<br/>Write failing test"]
    style red fill:#ffcccc
    verify_red{"Verify fails<br/>correctly"}
    green["GREEN<br/>Minimal code"]
    style green fill:#ccffcc
    verify_green{"Verify passes<br/>All green"}
    refactor["REFACTOR<br/>Clean up"]
    style refactor fill:#ccccff
    next_step(["Next"])

    red --> verify_red
    verify_red -->|yes| green
    verify_red -->|"wrong<br/>failure"| red
    green --> verify_green
    verify_green -->|yes| refactor
    verify_green -->|no| green
    refactor -->|"stay<br/>green"| verify_green
    verify_green --> next_step
    next_step --> red

RED - Write Failing Test

Write one minimal test showing what should happen.

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test. Don't add features, refactor other code, or "improve" beyond the test.