A Babel.js developer is responsible for converting ECMAScript + code into a backward-compatible version of JavaScript to ensure compatibility across different browsers. They work on optimizing code, debugging, and ensuring the seamless execution of JavaScript applications. They also provide solutions to complex JavaScript problems, ensure code maintainability, and stay updated with the latest ECMAScript specifications. They often work with development teams, collaborating on large scale projects to integrate Babel.js transpiling and polyfilling solutions, thus enhancing the performance and user experience of web applications. Their expertise lies in understanding and implementing Babel.js's customization features like plugins and presets.