Standard Callback Pattern in Node Js

AIM: To demonstrate the use of Standard callback pattern 
Objective: Node js callback pattern function callback 
Theory: Callback is an asynchronous equivalent for a function. A callback function is called at the completion of a given task. Node makes heavy use of callbacks. All the APIs of Node are written in such a way that they support callbacks. 
For example, a function to read a file may start reading file and return the control to the execution environment immediately so that the next instruction can be executed. 
Explanation: Once file I/O is complete, it will call the callback function while passing the callback function, the content of the file as a parameter. So there is no blocking or wait for File I/O. This makes Node.js highly scalable, as it can process a high number of requests without waiting for any function to return results. Create a text file named input.txt with the following content :
Create a js file named main.js with the following code :

var fs = require("fs");
var data = fs.readFileSync('input.txt');
console.log(data.toString());
console.log("Program Ended");
Now run the main.js to see the result −
$ node main.js
Verify the Output.