TDD

Drive implementation through the red-green-refactor cycle.

Workflow

1. Understand the behavior: read enough code to know what the change should do and where it belongs.
2. Write one failing test: the test describes the next behavior to add. It must fail for the right reason.
3. Make it pass: write the minimum code to pass the test. Nothing more.
4. Refactor: clean up while green. Improve naming, remove duplication, simplify structure. Run tests after each change.
5. Repeat: pick the next behavior and go back to step 2.

One test at a time. Each cycle is vertical — one test, one implementation, one refactor pass. Do not write multiple tests before implementing.

What to test