- You are not allowed to write any production code unless it is to make a failing unit test pass.
- You are not allowed to write any more of a unit test than is sufficient to fail; and compilation failures are failures.
- You are not allowed to write any more production code than is sufficient to pass the one failing unit test.
- Write a failing test. Stop writing the test as soon as it fails.
- Write the minimal production code required for the test to pass. Stop writing any code once the test passes.
- Refactor the test code and the production code without altering the functionality. All tests should pass.