With the advent of the web, the traditional JavaScript was intended to add simple functionality and minimal runtime in the browser. Thus, JavaScript was kept as a single-threaded scripting language. Now, just to get a brief idea regarding single-threaded model, let's consider the following diagram:

A single-threaded model creates a single Callstack in an execution context. In the preceding code, when the function getData() is invoked, the function is pushed in the stack for execution sequentially.

In the context of Node.js, JavaScript is the base scripting language, hence, Node.js is single-threaded. You might be asking, how does a single-threaded model help? Typical PHP, ASP.NET, Ruby, or Java-based servers follow a model where each client request results in the instantiation of a new thread or even a process.

When it comes to Node.js, requests are run on the same thread with shared resources. A question that is often asked is, what will be the advantage of using such a model? To understand this, we should understand the problem that Node.js tries to resolve. It tries to do asynchronous processing on a single thread to provide more performance and scalability for applications that are supposed to handle too much web traffic. Imagine web applications that handle millions of concurrent requests; if the server makes a new thread for handling each request that comes in, it will consume a lot of resources and we will end up trying to add more and more servers to increase the scalability of the application.

The single-threaded, asynchronous processing model has its advantage in the previous context, and you can process much more concurrent requests with fewer server-side resources. However, there is a downside to this approach; Node (by default) will not utilize the number of CPU cores available on the server it is running on, without using extra modules like pm2.