An AngularJS development agency ensures code quality by adhering to strict development standards and using automated testing tools. They often implement test-driven development (TDD) to catch bugs early and maintain high-quality code throughout the development process. Additionally, agencies follow best practices like modular coding, which makes the application more maintainable and scalable. A professional AngularJS development agency also conducts regular code reviews, ensuring the codebase remains clean, efficient, and free from technical debt. This focus on quality not only reduces the chances of errors but also improves the long-term maintainability of the application.