Node.js Callback Function

Node.js Callback Function

Node.js Callback Function : Asynchronism is one of the fundamental factor for Node.js to have become popular. And Callback is the realization of asynchronism for functions. Generally, in Node.js, most of the functions that work on resources have callback variants.

When an asynchronous function is called upon a resource for some task, the control is let immediately to continue with the subsequent statements after the function. The task on the resource would start in parallel. This helps Node.js continue with other tasks while the function is working with the resource. Once the task with the resource is completed, Node.js resumes with the callback function. Callback function is called with arguments : data object, result object and (or) error object containing information regarding the task.

Blocking Function: In contrast to asynchronous function, a synchronous function blocks the execution until the task on the resource is completed. Therefore synchronous function is also called as blocking function.

Node.js Nested Callback Function: If there are multiple operations to be done on the resource sequentially, and also has to be done asynchronously, you may nest the callback functions, one in another.

Now we shall see the working of a callback function in comparison with a blocking function, on reading (task) a file (resource).

• Example for Node.js Blocking Function
• Example for Node.js Callback Function
• Example for Node.js Nested Callback Function

Example 1 – Blocking Function

In this example, we will read a file “sample.txt” synchronously using readFileSync() method.

read-file-sync.js

var fs = require('fs');

// read file sample.txt
var data = fs.readFileSync('sample.txt');
console.log("Reading file completed : " + new Date().toISOString());

console.log("After readFileSync statement : " + new Date().toISOString());

Run this program using node in command-prompt/terminal, and you will get the following output.

Output

arjun@arjun-VPCEH26EN:~/nodejs$ node read-file-sync.js 
Reading file completed : 2017-10-19T12:21:40.103Z
After readFileSync statement : 2017-10-19T12:21:40.105Z

The After readFileSync statement  is always executed only after reading the file is completed. fs.readFileSync  is blocking the execution flow.

Example 2 – Node.js Callback Function

Following is an example node script which reads “sample.txt” file asynchronously, with the help of Node.js Callback Function.

read-file-async.js

var fs = require('fs');

// read file sample.txt
fs.readFile('sample.txt',
	// callback function that is called when reading file is done
	function(err, data) {		
		if (err) throw err;
		// data is a buffer containing file content
		console.log("Reading file completed : " + new Date().toISOString());
});

console.log("After readFile asynchronously : " + new Date().toISOString());

Run this program using node in command-prompt/terminal, and you will get the following output.

Output

arjun@arjun-VPCEH26EN:~/nodejs$ node read-file-async.js 
After readFile asynchronously : 2017-10-19T12:25:36.982Z
Reading file completed : 2017-10-19T12:25:36.987Z

You may observe that even after executing console.log(“After readFile asynchronously  statement, it took around 5ms to complete reading the file. fs.readFile(‘sample.txt’, callback function{..}) has not blocked the execution, instead a new process is started in parallel with the main control flow, to read the file (perform the task on resource).

Example 3 – Node.js Nested Callback Function

To demonstrate Node.js Nested Callback Function, we shall consider a scenario of renaming a file and then deleting it using asynchronous functions.

nodejs-nested-callback.js

var fs = require('fs');

fs.rename('sample.txt', 'sample_old.txt',
	// 1st call back function
	function (err) {
		if (err) throw err;
		console.log('File Renamed.');
		fs.unlink('sample_old.txt',
			// 2nd call back function
			function (err) {
				if (err) throw err;
				console.log('File Deleted.');
			}
		); 
	}
);

Run this program using node in command-prompt/terminal, and you will get the following output.

Output

arjun@arjun-VPCEH26EN:~/nodejs$ node nodejs-nested-callback.js 
File Renamed.
File Deleted.

Conclusion

In this Node.js Tutorial – Node.js Callback Function, we have learnt the execution flow of Callback functions and how they are non-blocking, and also how to use nested callback functions with examples.