From http://www.w3schools.com (Copyright Refsnes Data)

AJAX Suggest Example

We have seen that AJAX can be used to create more interactive applications.

AJAX Suggest Example

In the AJAX example below we will demonstrate how a web page can communicate with a web server online as a user enters data into a standard HTML form.

Type a Name in the Box Below

Suggestions:

Example Explained - The HTML Form

The form above has the following HTML code:

<form> 
First Name:
<input type="text" id="txt1"
onkeyup="showHint(this.value)">
</form>

<p>Suggestions: <span id="txtHint"></span></p>

As you can see it is just a simple HTML form with an input field called "txt1".

An event attribute for the input field defines a function to be triggered by the onkeyup event.

The paragraph below the form contains a span called "txtHint". The span is used as a placeholder for data retrieved from the web server.

When the user inputs data, a function called "showHint()" is executed. The execution of the function is triggered by the "onkeyup" event. In other words: Each time the user moves his finger away from a keyboard key inside the input field, the function showHint is called.

Example Explained - The showHint() Function

The showHint() function is a very simple JavaScript function placed in the <head> section of the HTML page.

The function contains the following code:

function showHint(str)
{
if (str.length==0)
  { 
  document.getElementById("txtHint").innerHTML="";
  return;
  }
xmlHttp=GetXmlHttpObject();
if (xmlHttp==null)
  {
  alert ("Your browser does not support AJAX!");
  return;
  } 
var url="gethint.asp";
url=url+"?q="+str;
url=url+"&sid="+Math.random();
xmlHttp.onreadystatechange=stateChanged;
xmlHttp.open("GET",url,true);
xmlHttp.send(null);
}

The function executes every time a character is entered in the input field.

If there is some input in the text field (str.length > 0) the function executes the following:

Defines the url (filename) to send to the server
Adds a parameter (q) to the url with the content of the input field
Adds a random number to prevent the server from using a cached file
Creates an XMLHTTP object, and tells the object to execute a function called stateChanged when a change is triggered
Opens the XMLHTTP object with the given url.
Sends an HTTP request to the server

If the input field is empty, the function simply clears the content of the txtHint placeholder.

Example Explained - The GetXmlHttpObject() Function

The example above calls a function called GetXmlHttpObject().

The purpose of the function is to solve the problem of creating different XMLHTTP objects for different browsers.

The function is listed below:

function GetXmlHttpObject()
{
var xmlHttp=null;
try
  {
  // Firefox, Opera 8.0+, Safari
  xmlHttp=new XMLHttpRequest();
  }
catch (e)
  {
  // Internet Explorer
  try
    {
    xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
    }
  catch (e)
    {
    xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
  }
return xmlHttp;
}

Example Explained - The stateChanged() Function

The stateChanged() function contains the following code:

function stateChanged() 
{ 
if (xmlHttp.readyState==4)
{ 
document.getElementById("txtHint").innerHTML=xmlHttp.responseText;
}
}

The stateChanged() function executes every time the state of the XMLHTTP object changes.

When the state changes to 4 ("complete"), the content of the txtHint placeholder is filled with the response text.