So what’s the difference between Tomcat and Node.js?

They’re different kinds of things. Node.js provides an environment for running JavaScript outside a browser. Tomcat is a web server and servlet container that runs on the Java runtime. The comparison is closer to Node.js versus the Java runtime on one side, and Tomcat versus a Node web server on the other.

With Node, your application can create an HTTP server directly, or use a framework such as Express to handle routing and requests. In either case, a Node process is running the code that listens for and responds to requests.

A simple way to picture it: the runtime is what lets your program run, while the server is a program that listens for network requests and sends responses. Node.js lets JavaScript run on a machine or server. Your JavaScript application can then start a server using Node’s built-in HTTP features or a framework.

Tomcat is more specialized. It runs Java web components and provides web-server functionality. So the labels describe different roles, and a single product or process can cover more than one role.

For a basic stack diagram, you could write: browser sends a request, a Node.js application receives it, application code decides what to do, and it returns a response. A framework like Express can make the request-handling part easier, but it still runs on Node.js.

Node is often called a runtime because that is its core job. Calling it a server can still make sense informally when people mean the Node process running a web server. The wording depends on whether they mean the runtime or the application built with it.

One small correction to the original mental model: Tomcat itself also runs on Java, so it depends on a runtime rather than replacing one. Tomcat adds web-specific services, including handling servlet requests. Node.js gives you the JavaScript runtime and networking tools, but your app supplies or chooses the web-server layer.

Also, JavaScript’s usual execution model does not mean Node can only handle one request at a time. Its asynchronous I/O is designed to manage many waiting network operations. Whether a particular app handles its workload well depends on its code and setup.