A JavaScript developer is responsible for implementing web-based client and server-side applications using JavaScript programming language. They design and develop user interfaces with JavaScript and related frameworks such as React, Angular or Vue.js. They also use Node.js for server-side scripting to create dynamic web page content before the page is sent to the user's web browser. Additionally, they debug and troubleshoot code, optimize applications for speed and scalability, and ensure cross-browser compatibility. They work closely with other developers and stakeholders to meet project requirements, improve user experience, and maintain code quality and best practices.