Let us learn Node.js – Part -III

Siva Kumar | August 17, 2018 | nodejs | No Comments

Node Package Manager provides two main functionalities:

It provides online repositories for node.js packages/modules which are searchable on

search.nodejs.org

It also provides command line utility to install Node.js packages, do version management

and dependency management of

Node.js packages.

The npm comes bundled with Node.js installables in versions after that v0.6.3. You can check

the version by opening Node.js command prompt and typing the following command:

npm version

What is Callback?

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. 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.

Blocking Code Example

Create a text file named input.txt with the following content −

This file contains the text for testing callback example

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

Output.

This file contains the text for testing callback example

Program Ended

Non-Blocking Code Example

Create a text file named input.txt with the following content.

This file contains the text for testing callback example

Update main.js to have the following code −

var fs = require("fs");

fs.readFile('input.txt', function (err, data) {
 
  if (err) return console.error(err);

   console.log(data.toString());
});

console.log("Program Ended");

Now run the main.js to see the result −

$ node main.js

Output.

Program Ended

This file contains the text for testing callback example

These two examples explain the concept of blocking and non-blocking calls.

The first example shows that the program blocks until it reads the file and then only it proceeds to end the program.
The second example shows that the program does not wait for file reading and proceeds to print “Program Ended” and at the same time, the program without blocking continues reading the file.

Thus, a blocking program executes very much in sequence. From the programming point of view, it is easier to implement the logic but non-blocking programs do not execute in sequence. In case a program needs to use any data to be processed, it should be kept within the same block to make it sequential execution.