A 'js' file is a plain text file containing JavaScript code. JavaScript is a high-level, interpreted programming language primarily used to add interactivity and dynamic behavior to websites. It is one of the core technologies of the World Wide Web, alongside HTML and CSS. JavaScript code within a '.js' file can be embedded directly into HTML pages or linked externally. When a web browser encounters JavaScript code, it interprets and executes it, allowing for manipulation of the Document Object Model (DOM), handling user events, making asynchronous requests to servers (AJAX), and creating rich, interactive user interfaces. JavaScript is also used in server-side environments (Node.js), desktop applications (Electron), and mobile applications (React Native, Ionic). The code within a '.js' file typically consists of variables, functions, objects, and control flow statements that define the logic and functionality of a web page or application. These files are essential for creating dynamic and engaging web experiences.