What is TDD?
TDD stands for Test Driven Development. The process is simple:

1. Before you write implementation code, write some code that proves that the implementation works or fails. Watch the test fail before moving to the next step (this is how we know that a passing test is not a false positive — how we test our tests).
2. Write the implementation code and watch the test pass.
3. Refactor if needed. You should feel confident refactoring your code now that you have a test to tell you if you’ve broken something.