A JavaScript file contains code written in the JavaScript programming language. JavaScript is a high-level, interpreted programming language primarily used to add interactivity to websites. It enables dynamic content updates, user interface enhancements, and asynchronous communication with servers. JavaScript files are typically embedded within HTML documents or linked externally. They are executed by web browsers on the client-side, allowing for real-time manipulation of the Document Object Model (DOM) and creation of rich user experiences. Beyond web development, JavaScript is also used in server-side environments (Node.js), desktop applications (Electron), and mobile app development (React Native, Ionic). JavaScript files are essential for modern web development, enabling developers to create responsive, interactive, and dynamic web applications. They are plain text files that can be edited with any text editor or specialized code editor.