A Meteor.js developer designs, implements, and manages web applications using the Meteor.js framework, a full-stack JavaScript platform for developing modern web and mobile applications. They write server-side and client-side code, ensure optimal performance of applications, and troubleshoot and fix bugs. They also integrate other JavaScript-based technologies, like React and MongoDB. Additionally, they manage real-time updates and scalable features of applications, and collaborate with other developers and stakeholders to understand their requirements and translate them into functional software. They also need to stay updated with the latest industry trends for continuous improvement.