Comprehensive backend testing for REST APIs, databases, authentication, and business logic across Jest, Pytest, and Mocha.

• Covers unit tests for isolated functions, integration tests for API endpoints, and authentication/authorization flows with detailed examples for Express.js, FastAPI, and other frameworks
• Includes test environment setup, mocking strategies for external dependencies, and database isolation patterns using in-memory or separate test databases
• Provides step-by-step guidance on AAA pattern (Arrange-Act-Assert), test fixtures, and role-based access control testing with JWT tokens
• Enforces test isolation, clear naming conventions, and coverage thresholds (default 80%) to prevent shared state issues and ensure reliable test execution