One of the most powerful features of Node.js is that it is both event-driven and asynchronous. So, how does an asynchronous model work? Imagine you have a block of code and at some n^th line you have an operation that is time consuming. What happens to the lines that follow the n^th line while this code gets executed? In normal synchronous programming models, the lines that follow the n^th line will have to wait until the operation at that line completes. An asynchronous model handles this case differently.

Let us visualize this scenario with the help of the following code and diagram:

In the preceding case, the setTimeout() method is provided by JavaScript (Node.js) API. Hence, this method is recognized as synchronous and is executed in a different execution context. According to functionality to setTimeout() , it executes the callback function after a specified duration, in our case after three seconds.

Further, the current execution is never blocked for a process to complete. When Node.js API determines that the completion of an event has been fired, it will execute your callback function at that moment.

In a typical synchronous programming language, executing the preceding code will yield the following output: