A JavaScript React developer designs and implements user interface components for JavaScript-based web and mobile applications using the React library. They work on the full application lifecycle, including concept, design, test, release, and support stages. Their tasks involve creating reusable, effective, and scalable JavaScript codes. They also optimize components for maximum performance across devices and browsers, ensuring system consistency and stability. They often collaborate with back-end developers and web designers to improve usability and translate designs and wireframes into high-quality code. They may also update and maintain existing applications to ensure optimal performance.