A Johnny-five.js developer is responsible for creating and maintaining software applications using the Johnny-five JavaScript Robotics and IoT platform. They use this framework to control and program hardware devices such as robots, drones, or thermostats. Their duties also include integrating the hardware components with other software systems, troubleshooting issues, and improving system performance. Additionally, they may work on designing user interfaces for device control. They need to have strong problem-solving skills, a thorough understanding of JavaScript, and knowledge of hardware programming. They often collaborate with other developers, engineers, and stakeholders to ensure the systems meet specific needs.