What Is TDD and Why It Works

TDD Defined

Test-Driven Development (TDD) is a development practice with one deceptively simple rule: write the test before you write the code.

The cycle:

1. Write a test that describes a requirement (it will fail — the code doesn't exist yet)
2. Write the minimum code to make the test pass
3. Refactor the code without changing behavior (tests verify you didn't break anything)
4. Repeat

Why Write Tests First?

When you write tests after implementation, you unconsciously test what the code does rather than what it should do. The tests are biased toward the implementation.

When you write tests first, you are forced to think about requirements before implementation. This produces fundamentally different code:

• Testable by design — if you can't write a test for it, the code is probably too complex
• Minimal — you only write code that a test demands
• Behavior-focused — tests capture intent, not implementation

The Three Laws of TDD

1. You may not write production code until you have a failing test.
2. You may not write more of a test than is sufficient to fail.
3. You may not write more production code than is sufficient to pass the test.

TDD with AI Coding Tools

TDD and AI tools form a powerful combination:

1. You write the test (defines the requirement precisely)
2. AI implements the code (generates something that passes)
3. Tests verify correctness automatically

The test you write becomes the specification for the AI. The more precise and comprehensive your tests, the better the AI's implementation.

Key Takeaways

• TDD writes tests before code — forces clarity about requirements before implementation begins
• The three laws: no production code without a failing test, test only enough to fail, code only enough to pass
• TDD produces testable, minimal, behavior-focused code by design
• AI tools + TDD: you write the tests (the specification), the AI writes the implementation