A JavaScript developer designs, implements, and tests web applications using JavaScript programming language. They use JavaScript frameworks like React, Angular, or Vue.js to build user interfaces and single-page applications. They work closely with other developers, UX/UI designers, and stakeholders to translate user requirements into functional and appealing interactive applications. They ensure the technical feasibility of UI/UX designs, optimize applications for speed and scalability, and fix bugs. They also maintain an understanding of the latest web applications and programming practices, often contributing to documentation and code review. They must ensure their code meets accessibility and web standards.