React.js development companies ensure the quality of delivered projects through a combination of practices. Firstly, they conduct comprehensive code reviews, where experienced developers meticulously review the React.js codebase, identifying potential issues, bugs, and areas for improvement. Secondly, they implement automated testing, using frameworks like Jest or React Testing Library to perform unit tests, integration tests, and end-to-end tests, ensuring the reliability and stability of the application. Thirdly, they follow industry-standard coding conventions and best practices to maintain clean, readable, and maintainable code. Fourthly, they emphasize regular communication and collaboration with clients to understand their requirements, provide updates, and address any concerns throughout the development process. Additionally, React.js development companies often leverage continuous integration and deployment practices to ensure that the project is continuously built, tested, and deployed in an automated and efficient manner. Lastly, they may engage in post-launch support and maintenance to address any issues, provide updates, and ensure the long-term success of the React.js project.