How do engines work? Engines are complicated. But the basics are easy. The engine (embedded if it’s a browser) reads (“parses”) the script. Then it converts (“compiles”) the script to machine code. And then the machine code runs, pretty fast. The engine applies optimizations at each step of the process. It even watches the compiled script as it runs, analyzes the data that flows through it, and further optimizes the machine code based on that knowledge.
Add new HTML to the page, change the existing content, modify styles. React to user actions, run on mouse clicks, pointer movements, key presses. Send requests over the network to remote servers, download and upload files (so-called AJAX and COMET technologies). Get and set cookies, ask questions to the visitor, show messages. Remember the data on the client-side (“local storage”).