A Grunt.js developer is responsible for automating repetitive tasks to streamline the development process. They use Grunt.js, a JavaScript task runner, to automatically perform frequent tasks such as minification, compilation, unit testing, and linting. This enhances efficiency and reduces errors. The developer creates and manages Gruntfiles and plugins, which define and configure tasks and load Grunt plugins. They also debug issues within the Grunt environment, ensuring smooth operation. Furthermore, they may collaborate with other developers to integrate Grunt.js into broader project workflows. Hence, a Grunt.js developer significantly contributes to optimizing and maintaining the quality of the codebase.