Node.js Callbacks and Events: Understanding I/O and Event-Driven Programming, Slides of Web Programming and Technologies

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CALLBACKS, EVENTS

Dr. Adeel Ansari

TOPICS TO BE COVERED

HTTP Module

FS Module

OS Module

URL Module

Event Loops and Event Emitter Class

BLOCKING CODE EXAMPLE

Create a text file named input.txt with some text content.

Create a main.js file with the following code:

var fs = require("fs");

var data =

fs.readFileSync('input.txt');

console.log(data.toString());

console.log("Program Ended");

NON-BLOCKING CODE

EXAMPLE

Create a text file named input.txt with some text content.

Create a main.js file with 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");

HTTP MODULE

Dr. Adeel Ansari

RECALL THE HTTP CODE

The function passed into the http.createServer() has a req

argument that represents the request from the client, as an object

(http.IncomingMessage object).

This object has a property called "url" which holds the part of the

url that comes after the domain name:

var http = require('http');

http.createServer(function (req, res) {

res.writeHead( 200 , {'Content-Type': 'text/html'

res.write('Hello World!');

res.end();

}).listen( 8080 );

Only “/” gets

printed on the

webpage.

FS MODULE

Dr. Adeel Ansari

NODE.JS AS A FILE SERVER

The Node.js file system module allows you to work with the file

system on your computer.

To include the File System module, use the require() method:

var fs = require('fs');

Common use for the File System module:

Read files

Create files

Update files

Delete files

Rename files

CREATE FILES

The File System module has methods for creating new files:

fs.appendFile()

fs.open()

fs.writeFile()

The fs.appendFile() method appends specified content to a file. If

the file does not exist, the file will be created

CREATE A NEW FILE USING

THE APPENDFILE() METHOD:

var fs = require('fs');

fs.appendFile('mynewfile1.txt', 'Hello

Content!', function (err) {

if (err) throw err;

console.log('Saved!');

CREATE A NEW FILE USING

THE WRITEFILE() METHOD:

The fs.writeFile() method replaces the specified file and content if it

exists. If the file does not exist, a new file, containing the specified

content, will be created:

var fs = require('fs');

fs.writeFile('mynewfile3.txt', 'Hello

content!', function (err) {

if (err) throw err;

console.log('Saved!');

UPDATE FILES

The File System module has methods for updating files:

fs.appendFile()

fs.writeFile()

The fs.appendFile() method appends the specified content at the

end of the specified file:

var fs = require('fs');

fs.appendFile('mynewfile1.txt', ' This is my

text.', function (err) {

if (err) throw err;

console.log('Updated!');

DELETE FILES

To delete a file with the File System module, use the fs.unlink()

method.

The fs.unlink() method deletes the specified file:

var fs = require('fs');

fs.unlink('mynewfile2.txt', function (err) {

if (err) throw err;

console.log('File deleted!');

RENAME FILES

To rename a file with the File System module, use the fs.rename()

method.

The fs.rename() method renames the specified file:

var fs = require('fs');

fs.rename('mynewfile1.txt', 'myrenamedfile.txt', function (err

if (err) throw err;

console.log('File Renamed!');