Topic 4

Nodejs Official Website: Nodejs.org

Youtube Resources: Detailed and Brief

Getting Started with Nodejs

Reference for Topic 4 is taken from W3School/Nodejs

Once we have installed Node.js, let’s build our first web server. Make a new file server.js and write the following code in it.

/*server.js*/

const http = require("http");

const hostname = "127.0.0.1";
const port = 3000;

const server = http.createServer(function (req, res) {
  res.statusCode = 200;
  res.setHeader("Content-Type", "text/plain");
  res.end("Hello World\n");
});

server.listen(port, hostname, function () {
  console.log("Server running at http://" + hostname + ":" + port + "/");
});

Understanding Code

As we need http to create an http server we use require(‘http’) and pass it to a variable named http.

const http = require("http");

We also need to defined hostname and port number, here we use localHost i.e 127.0.0.1 and port number 3000 and assign this to the variables hostname and port, respectively.

const hostname = "127.0.0.1";
const port = 3000;

Next we create the http server using the createServer method.

const server = http.createServer(function (req, res) {
  res.statusCode = 200;
  res.setHeader("Content-Type", "text/plain");
  res.end("Hello World\n");
});

This created the server as well as a response having statusCode: 200, Content-Type header of plain text and and ends with the string Hello World. This is the response that the server can send to browser. the function has two parameters req and res which is the request from and response to the server, respectively.

We created the server, now we just have to assign it a hostname and port number.

server.listen(port, hostname, function () {
  console.log("Server running at http://" + hostname + ":" + port + "/");
});

Now run server.js file in terminal using command node server
Here, the server listens to localhost on port 3000 and prints “Server running at http://127.0.0.1:3000/” in command prompt.
Open a browser and enter url http://127.0.0.1:3000/. The browser will display Hello World message on the screen.

To understand how require() works visit this link

Node.js Modules

Consider modules to be the same as JavaScript libraries.
A set of functions you want to include in your application.
Node.js has a set of built-in modules which you can use without any further installation.
Modules are reusable functionality.

Importing modules

To import a module we use the require() function in node. Require function is globally available.

var http = require("http");

Now your application has access to the HTTP module, and is able to create a server.

Let’s see some of the features of modules in the examples below. Make global.js file and add the following code.

var path = require("path");
console.log(
  `Full path: ${__filename} and filename is ${path.basename(__filename)}`
);

Now run command- node global and you will see output as full path of the global.js file.
The path module provides us some methods to work with files/folders. The basename method returns the filename from the complete path. __filename is a variable that is available in all node file and it refers the complete path of the current file.

Create Your Own Modules

One can create their own modules and use in their applications.
Let’s try to make one.
Make a file, mymodule.js and add the follwing code in it.

exports.myDateTime = function () {
  return Date();
};

Use the exports keyword to make properties and methods available outside the module file.
] Now let’s include this module in our files. Open server.js file and include following code in it.

const http = require("http");
var dt = require("./mymodule"); //newline

const hostname = "127.0.0.1";
const port = 3000;

//create a server object:
const server = http.createServer(function (req, res) {
  res.statusCode = 200;
  res.setHeader("Content-Type", "text/plain");
  res.write("The date and time are currently: " + dt.myDateTime()); //new line //write a response to the client
  res.end("Hello World\n"); //end the response
});

server.listen(port, hostname, function () {
  console.log("Server running at http://" + hostname + ":" + port + "/");
}); //the server object listens on port 3000

Now run server using node in command line and see the output.

Built-in Http Module

The HTTP module can create an HTTP server that listens to server ports and gives a response back to the client. We have used the createServer() method to create an HTTP server (in server.js file)
The function passed into the http.createServer() method, will be executed when someone tries to access the computer on port 3000.

Read the Query String

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:

Now make a new file httpmodule.js and add the following code:

const http = require("http");

const hostname = "127.0.0.1";
const port = 3000;

const server = http.createServer(function (req, res) {
  res.writeHead(200, { "Content-Type": "text/html" });
  res.write(req.url);
  res.end();
});

server.listen(port, hostname, function () {
  console.log("Server running at http://" + hostname + ":" + port + "/");
});

Now run this file using command node httpmodule and try to run these urls http://localhost:3000/uplift and http://localhost:3000/program.

File System Module

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

var fs = require("fs");

Common use for the File System module:

Read files
Create files
Update files
Delete files
Rename files

Read Files

The fs.readFile() method is used to read files on your computer.

Assume we have the following HTML file (demofile.html):

<html>
  <body>
    <h1>My Header</h1>
    <p>My paragraph.</p>
  </body>
</html>

Create a readfile.js file that reads the HTML file, and return the content:

const http = require('http');
var fs = require('fs');

const hostname = '127.0.0.1';
const port = 3000;

const server = http.createServer(function(req, res) {

 fs.readFile('demofile.html', function(err, data) {
    res.writeHead(200, {'Content-Type': 'text/html'});
    res.write(data);
    return res.end();
}});

server.listen(port, hostname, function() {
  console.log('Server running at http://'+ hostname + ':' + port + '/');
});

Run the above file and see the result.

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!");
});

The fs.open() method takes a “flag” as the second argument, if the flag is “w” for “writing”, the specified file is opened for writing. If the file does not exist, an empty file is created:

Create a new, empty file using the open() method:

var fs = require("fs");

fs.open("mynewfile2.txt", "w", function (err, file) {
  if (err) throw err;
  console.log("Saved!");
});

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:

Create a new file using the writeFile() method:

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:
Append “This is my text.” to the end of the file “mynewfile1.txt”:

var fs = require("fs");

fs.appendFile("mynewfile1.txt", " This is my text.", function (err) {
  if (err) throw err;
  console.log("Updated!");
});

The fs.writeFile() method replaces the specified file and content:
Replace the content of the file “mynewfile3.txt”:

var fs = require("fs");

fs.writeFile("mynewfile3.txt", "This is my text", function (err) {
  if (err) throw err;
  console.log("Replaced!");
});

Delete Files

To delete a file with the File System module, use the fs.unlink() method.
The fs.unlink() method deletes the specified file:

Delete “mynewfile2.txt”:

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:

Rename “mynewfile1.txt” to “myrenamedfile.txt”:

var fs = require("fs");

fs.rename("mynewfile1.txt", "myrenamedfile.txt", function (err) {
  if (err) throw err;
  console.log("File Renamed!");
});

URL Module

The URL module splits up a web address into readable parts. Parse an address with the url.parse() method, and it will return a URL object with each part of the address as properties: For Example-

var url = require("url");
var adr = "http://localhost:8080/default.htm?year=2017&month=february";
var q = url.parse(adr, true);

console.log(q.host); //returns 'localhost:8080'
console.log(q.pathname); //returns '/default.htm'
console.log(q.search); //returns '?year=2017&month=february'

var qdata = q.query; //returns an object: { year: 2017, month: 'february' }
console.log(qdata.month); //returns 'february'

Node.js File Server

Create two html files and save them in the same folder as your node.js files. summer.html

<!DOCTYPE html>
<html>
  <body>
    <h1>Summer</h1>
    <p>I love the sun!</p>
  </body>
</html>

winter.html

<!DOCTYPE html>
<html>
  <body>
    <h1>Winter</h1>
    <p>I love the snow!</p>
  </body>
</html>

Create a Node.js file that opens the requested file and returns the content to the client. If anything goes wrong, throw a 404 error:

fileserver.js:

var http = require("http");
var url = require("url");
var fs = require("fs");

http
  .createServer(function (req, res) {
    var q = url.parse(req.url, true);
    var filename = "." + q.pathname;
    fs.readFile(filename, function (err, data) {
      if (err) {
        res.writeHead(404, { "Content-Type": "text/html" });
        return res.end("404 Not Found");
      }
      res.writeHead(200, { "Content-Type": "text/html" });
      res.write(data);
      return res.end();
    });
  })
  .listen(8080);

Events in Node.js

Every action on a computer is an event. Like when a connection is made or a file is opened. Objects in Node.js can fire events, like the readStream object fires events when opening and closing a file:

Example

var fs = require("fs");
var rs = fs.createReadStream("./demofile.txt");
rs.on("open", function () {
  console.log("The file is open");
});

Events Module

Node.js has a built-in module, called “Events”, where you can create-, fire-, and listen for- your own events.

To include the built-in Events module use the require() method. In addition, all event properties and methods are an instance of an EventEmitter object. To be able to access these properties and methods, create an EventEmitter object:

var events = require("events");
var eventEmitter = new events.EventEmitter();

The EventEmitter Object

You can assign event handlers to your own events with the EventEmitter object. In the example below we have created a function that will be executed when a “scream” event is fired. To fire an event, use the emit() method.

Example

var events = require("events");
var eventEmitter = new events.EventEmitter();

//Create an event handler:
var myEventHandler = function () {
  console.log("I hear a scream!");
};

//Assign the event handler to an event:
eventEmitter.on("scream", myEventHandler);

//Fire the 'scream' event:
eventEmitter.emit("scream");

Topic 4

👨🏻‍💻 TheNodeCourse - The Complete Learning Path for Nodejs Development.

Topic 4

Nodejs Official Website: Nodejs.org

Youtube Resources: Detailed and Brief

Getting Started with Nodejs

Reference for Topic 4 is taken from W3School/Nodejs

Understanding Code

Node.js Modules

Importing modules

Create Your Own Modules

Built-in Http Module

Read the Query String

File System Module

Read Files

Create Files

Update Files

Delete Files

Rename Files

URL Module

Node.js File Server

Events in Node.js

Events Module

The EventEmitter Object

Exercise

Try all these in your local system for the better understanding of nodejs. After all this you can try the Self-Evaluating Assignment.