A BabelJS developer uses Babel, a JavaScript compiler, to convert ECMAScript + code into a backwards-compatible version for older browsers or environments. They use Babel to transform syntax, polyfill features that are missing in target environments, and provide source code transformations. They ensure cross-browser compatibility of JavaScript applications, enabling the use of latest JavaScript features without waiting for client browser support. They also configure and maintain Babel according to project needs. They work closely with other developers, ensuring the codebase remains clean, efficient, and updated. Their role is crucial to maintaining the modernity and accessibility of JavaScript applications.