A Karma.js developer is responsible for implementing automated testing for JavaScript code. They design and write test cases using Karma, a test runner for JavaScript, to ensure code quality and functionality. They work closely with other developers to understand the functionality of the code, identify areas that need testing, and debug any issues that arise during testing. They also help in setting up the testing environment and integrating it with continuous integration tools. Their goal is to facilitate quicker, more efficient development by catching bugs early, minimizing errors, and promoting best coding practices.