Nodejs callback

A callback is a function that can be called on the completion of a given task to prevent blocking. As a result of it other code will run without waiting for a task to complete. This feature makes node.js highly scalable as it can process large numbers of requests without waiting for any function to return result.

Uses of callback methods:

Below are examples of blocking and non-blocking codes:

1. Create a text file with name test1.txt with the following content:

“Here we are explaining the node.js callbacks.”

2. Create a javascript file with name test2.js having the following code:

var fs = require("fs"); 
fs.readFile(‘test1.txt', function (err, data) { 
  if (err) return console.error(err); 
  console.log(data.toString()); 
}); 
console.log("Program Ended");

3. Now open the command prompt and execute the following command:

node test2.js

Blocking code output:

Non-Blocking code output:

As you can see in case of non-blocking code program does not wait for file reading but it just proceeded to print “Program Ended” and same time program without blocking continues reading the file.

Note: Using so many callbacks at once has a drawback known as callback hell