Practicing TDD
Write tests first, code second.
1. The TDD Flow and Cycle
- •Write a list of the test scenarios you want to cover
- •🔴 [RED] Write failing test: Turn exactly one item on the list into an actual, concrete, runnable test
- •🟢 [GREEN] Write minimal code to pass: Change the code to make the test (& all previous tests) pass (adding items to the list as you discover them)
- •🔵 [REFACTOR] Improve code quality: Optionally refactor to improve the implementation design
- •Until the list is empty, go back to #2
2. The Three Laws of TDD
- •Write production code only to make a failing test pass
- •Write only enough test to demonstrate failure
- •Write only enough code to make the test pass
3. RED Phase Principles
What to Write
| Focus | Example |
|---|---|
| Behavior | "should add two numbers" |
| Edge cases | "should handle empty input" |
| Error states | "should throw for invalid data" |
RED Phase Rules
- •Test must fail first
- •Test name describes expected behavior
- •One assertion per test (ideally)
4. GREEN Phase Principles
Minimum Code
| Principle | Meaning |
|---|---|
| YAGNI | You Aren't Gonna Need It |
| Simplest thing | Write the minimum to pass |
| No optimization | Just make it work |
GREEN Phase Rules
- •Don't write unneeded code
- •Don't optimize yet
- •Pass the test, nothing more
5. REFACTOR Phase Principles
What to Improve
| Area | Action |
|---|---|
| Duplication | Extract common code |
| Naming | Make intent clear |
| Structure | Improve organization |
| Complexity | Simplify logic |
REFACTOR Rules
- •All tests must stay green
- •Small incremental changes
- •Commit after each refactor
6. AAA Pattern
Every test follows:
| Step | Purpose |
|---|---|
| Arrange | Set up test data |
| Act | Execute code under test |
| Assert | Verify expected outcome |
7. When to Use TDD
| Scenario | TDD Value |
|---|---|
| New feature | High |
| Bug fix | High (write test first) |
| Complex logic | High |
| Exploratory | Low (spike, then TDD) |
| UI layout | Low |
8. Test Prioritization
| Priority | Test Type |
|---|---|
| 1 | Happy path |
| 2 | Error cases |
| 3 | Edge cases |
| 4 | Performance |
9. Anti-Patterns
| ❌ Don't | ✅ Do |
|---|---|
| Skip the RED phase | Watch test fail first |
| Write tests after | Write tests before |
| Over-engineer initial | Keep it simple |
| Multiple asserts | One behavior per test |
| Test implementation | Test behavior |
10. AI-Augmented TDD
Multi-Agent Pattern
| Agent | Role |
|---|---|
| Agent A | Write failing tests (RED) |
| Agent B | Implement to pass (GREEN) |
| Agent C | Optimize (REFACTOR) |
Remember: The test is the specification. If you can't write a test, you don't understand the requirement.