A Charts.js developer specializes in using the Charts.js library to create interactive, flexible, and customizable charts for web applications. They handle data visualization, converting raw data into visually appealing charts such as bar, line, pie, or scatter charts. They ensure these charts are responsive and compatible across different browsers and devices. Additionally, they work on integrating these charts with other parts of the web application, ensuring seamless interaction between the user interface and the backend. They also perform debugging, optimization, and maintenance of these charts, ensuring they function correctly and efficiently.