Wednesday, December 30, 2015

Improve the knowledge about Javascript

JavaScript is a lightweight, interpreted programming language. It is designed for creating network-centric applications. It is complimentary to and integrated with Java. JavaScript is very easy to implement because it is integrated with HTML. It is open and cross-platform.

Audience

This tutorial has been prepared for JavaScript beginners to help them understand the basic functionality of JavaScript to build dynamic web pages and web applications.

Prerequisites

For this tutorial, it is assumed that the reader have a prior knowledge of HTML coding. It would help if the reader had some prior exposure to object-oriented programming concepts and a general idea on creating online applications.

Execute JavaScript Online

For most of the examples given in this tutorial you will find Try it option, so just make use of this option to execute your JavaScript programs at the spot and enjoy your learning.

Try following example using Try it option available at the top right corner of the below sample code box −

<html>
   <body>
   
      <script language="javascript" type="text/javascript">
         <!--
            document.write("Hello World!")
         //-->
      </script>
      
   </body>
</html>


What is JavaScript ?

Javascript is a dynamic computer programming language. It is lightweight and most commonly used as a part of web pages, whose implementations allow client-side script to interact with the user and make dynamic pages. It is an interpreted programming language with object-oriented capabilities.

JavaScript was first known as LiveScript, but Netscape changed its name to JavaScript, possibly because of the excitement being generated by Java. JavaScript made its first appearance in Netscape 2.0 in 1995 with the nameLiveScript. The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers.

The ECMA-262 Specification defined a standard version of the core JavaScript language.


JavaScript is a lightweight, interpreted programming language.


Designed for creating network-centric applications.


Complementary to and integrated with Java.


Complementary to and integrated with HTML.


Open and cross-platform



Client-side JavaScript

Client-side JavaScript is the most common form of the language. The script should be included in or referenced by an HTML document for the code to be interpreted by the browser.

It means that a web page need not be a static HTML, but can include programs that interact with the user, control the browser, and dynamically create HTML content.

The JavaScript client-side mechanism provides many advantages over traditional CGI server-side scripts. For example, you might use JavaScript to check if the user has entered a valid e-mail address in a form field.

The JavaScript code is executed when the user submits the form, and only if all the entries are valid, they would be submitted to the Web Server.

JavaScript can be used to trap user-initiated events such as button clicks, link navigation, and other actions that the user initiates explicitly or implicitly.

Advantages of JavaScript

The merits of using JavaScript are −


Less server interaction − You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.


Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.


Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.


Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.



Limitations of JavaScript

We cannot treat JavaScript as a full-fledged programming language. It lacks the following important features −


Client-side JavaScript does not allow the reading or writing of files. This has been kept for security reason.


JavaScript cannot be used for networking applications because there is no such support available.


JavaScript doesn't have any multithreading or multiprocessor capabilities.



Once again, JavaScript is a lightweight, interpreted programming language that allows you to build interactivity into otherwise static HTML pages.

JavaScript Development Tools

One of major strengths of JavaScript is that it does not require expensive development tools. You can start with a simple text editor such as Notepad. Since it is an interpreted language inside the context of a web browser, you don't even need to buy a compiler.

To make our life simpler, various vendors have come up with very nice JavaScript editing tools. Some of them are listed here −


Microsoft FrontPage − Microsoft has developed a popular HTML editor called FrontPage. FrontPage also provides web developers with a number of JavaScript tools to assist in the creation of interactive websites.


Macromedia Dreamweaver MX − Macromedia Dreamweaver MX is a very popular HTML and JavaScript editor in the professional web development crowd. It provides several handy prebuilt JavaScript components, integrates well with databases, and conforms to new standards such as XHTML and XML.


Macromedia HomeSite 5 − HomeSite 5 is a well-liked HTML and JavaScript editor from Macromedia that can be used to manage personal websites effectively.



Where is JavaScript Today ?

The ECMAScript Edition 5 standard will be the first update to be released in over four years. JavaScript 2.0 conforms to Edition 5 of the ECMAScript standard, and the difference between the two is extremely minor.

Today, Netscape's JavaScript and Microsoft's JScript conform to the ECMAScript standard, although both the languages still support the features that are not a part of the standard.

Monday, November 9, 2015

Improve the knowledge about AJAX - Database Operations

To clearly illustrate how easy it is to access information from a database using AJAX, we are going to build MySQL queries on the fly and display the results on "ajax.html". But before we proceed, let us do the ground work. Create a table using the following command.

NOTE: We are assuming you have sufficient privilege to perform the following MySQL operations

CREATE TABLE 'ajax_example' (
   'name' varchar(50) NOT NULL,
   'age' int(11) NOT NULL,
   'sex' varchar(1) NOT NULL,
   'wpm' int(11) NOT NULL,
   PRIMARY KEY  ('name')
)

Now dump the following data into this table using the following SQL statements:

INSERT INTO 'ajax_example' VALUES ('Jerry', 120, 'm', 20);
INSERT INTO 'ajax_example' VALUES ('Regis', 75, 'm', 44);
INSERT INTO 'ajax_example' VALUES ('Frank', 45, 'm', 87);
INSERT INTO 'ajax_example' VALUES ('Jill', 22, 'f', 72);
INSERT INTO 'ajax_example' VALUES ('Tracy', 27, 'f', 0);
INSERT INTO 'ajax_example' VALUES ('Julie', 35, 'f', 90);

Client Side HTML File

Now let us have our client side HTML file, which is ajax.html, and it will have the following code:

<html>
<body>
<script language="javascript" type="text/javascript">
<!-- 
//Browser Support Code
function ajaxFunction(){
   var ajaxRequest;  // The variable that makes Ajax possible!
   try{
   
      // Opera 8.0+, Firefox, Safari
      ajaxRequest = new XMLHttpRequest();
   }catch (e){
      
      // Internet Explorer Browsers
      try{
         ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
      }catch (e) {
         
         try{
            ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
         }catch (e){
         
            // Something went wrong
            alert("Your browser broke!");
            return false;
         }
      }
   }
   
   // Create a function that will receive data
   // sent from the server and will update
   // div section in the same page.
   ajaxRequest.onreadystatechange = function(){
   
      if(ajaxRequest.readyState == 4){
         var ajaxDisplay = document.getElementById('ajaxDiv');
         ajaxDisplay.innerHTML = ajaxRequest.responseText;
      }
   }
   
   // Now get the value from user and pass it to
   // server script.
   var age = document.getElementById('age').value;
   var wpm = document.getElementById('wpm').value;
   var sex = document.getElementById('sex').value;
   var queryString = "?age=" + age ;
   
   queryString +=  "&wpm=" + wpm + "&sex=" + sex;
   ajaxRequest.open("GET", "ajax-example.php" + queryString, true);
   ajaxRequest.send(null); 
}
//-->
</script>

<form name='myForm'>

   Max Age: <input type='text' id='age' /> <br />
   Max WPM: <input type='text' id='wpm' /> <br />
   Sex: 
   <select id='sex'>
      <option value="m">m</option>
      <option value="f">f</option>
   </select>
   <input type='button' onclick='ajaxFunction()' value='Query MySQL'/>
   
</form>
<div id='ajaxDiv'>Your result will display here</div>
</body>
</html>

NOTE: The way of passing variables in the Query is according to HTTP standard and have formA.

URL?variable1=value1;&variable2=value2;

The above code will give you a screen as given below:

NOTE: This is dummy screen and would not work

Your result will display here in this section after you have made your entry.

NOTE: This is a dummy screen.

Server Side PHP File

Your client-side script is ready. Now, we have to write our server-side script, which will fetch age, wpm, and sex from the database and will send it back to the client. Put the following code into the file "ajax-example.php".

<?php
$dbhost = "localhost";
$dbuser = "dbusername";
$dbpass = "dbpassword";
$dbname = "dbname";
 
//Connect to MySQL Server
mysql_connect($dbhost, $dbuser, $dbpass);
 
//Select Database
mysql_select_db($dbname) or die(mysql_error());
 
// Retrieve data from Query String
$age = $_GET['age'];
$sex = $_GET['sex'];
$wpm = $_GET['wpm'];
 
// Escape User Input to help prevent SQL Injection
$age = mysql_real_escape_string($age);
$sex = mysql_real_escape_string($sex);
$wpm = mysql_real_escape_string($wpm);
 
//build query
$query = "SELECT * FROM ajax_example WHERE sex = '$sex'";

if(is_numeric($age))
   $query .= " AND age <= $age";

if(is_numeric($wpm))
   $query .= " AND wpm <= $wpm";
 
//Execute query
$qry_result = mysql_query($query) or die(mysql_error());

//Build Result String
$display_string = "<table>";
$display_string .= "<tr>";
$display_string .= "<th>Name</th>";
$display_string .= "<th>Age</th>";
$display_string .= "<th>Sex</th>";
$display_string .= "<th>WPM</th>";
$display_string .= "</tr>";

// Insert a new row in the table for each person returned
while($row = mysql_fetch_array($qry_result)){
   $display_string .= "<tr>";
   $display_string .= "<td>$row[name]</td>";
   $display_string .= "<td>$row[age]</td>";
   $display_string .= "<td>$row[sex]</td>";
   $display_string .= "<td>$row[wpm]</td>";
   $display_string .= "</tr>";
}

echo "Query: " . $query . "<br />";
$display_string .= "</table>";

echo $display_string;
?>

Now try by entering a valid value (e.g., 120) in Max Age or any other box and then click Query MySQL button.

Your result will display here in this section after you have made your entry.

If you have successfully completed this lesson, then you know how to use MySQL, PHP, HTML, and Javascript in tandem to write AJAX applications.

Improve the knowledge about AJAX - XMLHttpRequest

The XMLHttpRequest object is the key to AJAX. It has been available ever since Internet Explorer 5.5 was released in July 2000, but was not fully discovered until AJAX and Web 2.0 in 2005 became popular.

XMLHttpRequest (XHR) is an API that can be used by JavaScript, JScript, VBScript, and other web browser scripting languages to transfer and manipulate XML data to and from a webserver using HTTP, establishing an independent connection channel between a webpage's Client-Side and Server-Side.

The data returned from XMLHttpRequest calls will often be provided by back-end databases. Besides XML, XMLHttpRequest can be used to fetch data in other formats, e.g. JSON or even plain text.

You already have seen a couple of examples on how to create an XMLHttpRequest object.

Listed below is listed are some of the methods and properties that you have to get familiar with.

XMLHttpRequest Methods

abort()
Cancels the current request.
getAllResponseHeaders()
Returns the complete set of HTTP headers as a string.
getResponseHeader( headerName )
Returns the value of the specified HTTP header.
open( method, URL )

open( method, URL, async )

open( method, URL, async, userName )

open( method, URL, async, userName, password )

Specifies the method, URL, and other optional attributes of a request.

The method parameter can have a value of "GET", "POST", or "HEAD". Other HTTP methods, such as "PUT" and "DELETE" (primarily used in REST applications) may be possible.

The "async" parameter specifies whether the request should be handled asynchronously or not. "true" means that the script processing carries on after the send() method without waiting for a response, and "false" means that the script waits for a response before continuing script processing.
send( content )
Sends the request.
setRequestHeader( label, value )

Adds a label/value pair to the HTTP header to be sent.

XMLHttpRequest Properties

onreadystatechange
An event handler for an event that fires at every state change.
readyState
The readyState property defines the current state of the XMLHttpRequest object.

The following table provides a list of the possible values for the readyState property:

State Description

0 The request is not initialized.

1 The request has been set up.

2 The request has been sent.

3 The request is in process.

4 The request is completed.

readyState = 0 After you have created the XMLHttpRequest object, but before you have called the open() method.

readyState = 1 After you have called the open() method, but before you have called send().

readyState = 2 After you have called send().

readyState = 3 After the browser has established a communication with the server, but before the server has completed the response.

readyState = 4 After the request has been completed, and the response data has been completely received from the server.
responseText
Returns the response as a string.
responseXML
Returns the response as XML. This property returns an XML document object, which can be examined and parsed using the W3C DOM node tree methods and properties.
status
Returns the status as a number (e.g., 404 for "Not Found" and 200 for "OK").
statusText
Returns the status as a string (e.g., "Not Found" or "OK").

State	Description
0	The request is not initialized.
1	The request has been set up.
2	The request has been sent.
3	The request is in process.
4	The request is completed.

Improve the knowledge about AJAX Action

Steps of AJAX Operation

A client event occurs.
An XMLHttpRequest object is created.
The XMLHttpRequest object is configured.
The XMLHttpRequest object makes an asynchronous request to the Webserver.
The Webserver returns the result containing XML document.
The XMLHttpRequest object calls the callback() function and processes the result.
The HTML DOM is updated.

Let us take these steps one by one.

A Client Event Occurs

A JavaScript function is called as the result of an event.
Example: validateUserId() JavaScript function is mapped as an event handler to an onkeyup event on input form field whose id is set to"userid"
<input type="text" size="20" id="userid" name="id" onkeyup="validateUserId();">.

The XMLHttpRequest Object is Created

var ajaxRequest;  // The variable that makes Ajax possible!
function ajaxFunction(){
   try{
      
      // Opera 8.0+, Firefox, Safari
      ajaxRequest = new XMLHttpRequest();
   }catch (e){
   
      // Internet Explorer Browsers
      try{
         ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
      }catch (e) {
      
         try{
            ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
         }catch (e){
      
            // Something went wrong
            alert("Your browser broke!");
            return false;
         }
      }
   }
}

The XMLHttpRequest Object is Configured

In this step, we will write a function that will be triggered by the client event and a callback function processRequest() will be registered.

function validateUserId() {
   ajaxFunction();
   
   // Here processRequest() is the callback function.
   ajaxRequest.onreadystatechange = processRequest;
   
   if (!target) target = document.getElementById("userid");
   var url = "validate?id=" + escape(target.value);
   
   ajaxRequest.open("GET", url, true);
   ajaxRequest.send(null);
}

Making Asynchronous Request to the Webserver

Source code is available in the above piece of code. Code written in bold typeface is responsible to make a request to the webserver. This is all being done using the XMLHttpRequest object ajaxRequest.

function validateUserId() {
   ajaxFunction();
   
   // Here processRequest() is the callback function.
   ajaxRequest.onreadystatechange = processRequest;
   
   if (!target) target = document.getElementById("userid");
   var url = "validate?id=" + escape(target.value);
   
   ajaxRequest.open("GET", url, true);
   ajaxRequest.send(null);
}

Assume you enter Zara in the userid box, then in the above request, the URL is set to "validate?id=Zara".

Webserver Returns the Result Containing XML Document

You can implement your server-side script in any language, however its logic should be as follows.

Get a request from the client.
Parse the input from the client.
Do required processing.
Send the output to the client.

If we assume that you are going to write a servlet, then here is the piece of code.

public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException 
{
   String targetId = request.getParameter("id");
   
   if ((targetId != null) && !accounts.containsKey(targetId.trim()))
   {
      response.setContentType("text/xml");
      response.setHeader("Cache-Control", "no-cache");
      response.getWriter().write("true");
   }
   else
   {
      response.setContentType("text/xml");
      response.setHeader("Cache-Control", "no-cache");
      response.getWriter().write("false");
   }
}

Callback Function processRequest() is Called

The XMLHttpRequest object was configured to call the processRequest() function when there is a state change to the readyState of the XMLHttpRequestobject. Now this function will receive the result from the server and will do the required processing. As in the following example, it sets a variable message on true or false based on the returned value from the Webserver.

 
function processRequest() {
   if (req.readyState == 4) {
      if (req.status == 200) {
         var message = ...;
...
}

The HTML DOM is Updated

This is the final step and in this step, your HTML page will be updated. It happens in the following way:

JavaScript gets a reference to any element in a page using DOM API.
The recommended way to gain a reference to an element is to call.

document.getElementById("userIdMessage"), 
// where "userIdMessage" is the ID attribute 
// of an element appearing in the HTML document

JavaScript may now be used to modify the element's attributes; modify the element's style properties; or add, remove, or modify the child elements. Here is an example:

<script type="text/javascript">
<!--
function setMessageUsingDOM(message) {
   var userMessageElement = document.getElementById("userIdMessage");
   var messageText;
   
   if (message == "false") {
      userMessageElement.style.color = "red";
      messageText = "Invalid User Id";
   }
   else 
   {
      userMessageElement.style.color = "green";
      messageText = "Valid User Id";
   }
   
   var messageBody = document.createTextNode(messageText);
   
   // if the messageBody element has been created simple 
   // replace it otherwise append the new element
   if (userMessageElement.childNodes[0]) {
      userMessageElement.replaceChild(messageBody, userMessageElement.childNodes[0]);
   } 
   else
   {
      userMessageElement.appendChild(messageBody);
   }
}
-->
</script>
<body>
<div id="userIdMessage"><div>
</body>

If you have understood the above-mentioned seven steps, then you are almost done with AJAX. In the next chapter, we will see XMLHttpRequest object in more detail.