Unit testing best practices TDD

Write effective unit tests with TDD approach. TDD cycle: 1. Red (write failing test). 2. Green (write minimal code to pass). 3. Refactor (improve code while keeping tests green). Best practices: Test one thing per test. Use AAA pattern (Arrange, Act, Assert). Descriptive test names (should_returnTrue_when_inputIsValid). Mock external dependencies. Aim for 80%+ code coverage. Fast tests (<1 sec). Independent tests (no order dependency). Use test fixtures for setup. Frameworks: Jest, Pytest, JUnit. Benefits: confidence in changes, living documentation, better design.