XMLHttpRequest Object (1)

What is the XMLHttpRequest Object? The XMLHttpRequest object provides a way to communicate with a server after a web page has loaded. The XMLHttpRequest object is supported in all modern browsers. With the XMLHttpRequest object, you can:

• Update a web page with new data without reloading the page
• Request data from a server after the page has loaded
• Receive data from a server after the page has loaded
• Send data to a server in the background

In Javascript, to Create an XMLHttpRequest object, we use a single line.

In Internet Explorer 5 and 6:

var xmlhttp=new ActiveXObject("Microsoft.XMLHTTP")

In all modern browsers (including IE7):

var xmlhttp=new XMLHttpRequest()

For example

<script type="text/Javascript">
var xmlhttp;
function loadXMLDoc(url) {
xmlhttp=null;
if (window.XMLHttpRequest) {// code for all new browsers
xmlhttp=new XMLHttpRequest();
} else if (window.ActiveXObject) {// code for IE5 and IE6
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
if (xmlhttp!=null) {
xmlhttp.onreadystatechange=state_Change;
xmlhttp.open("GET",url,true);
xmlhttp.send(null);
} else {
alert("Your browser does not support XMLHTTP.");
}
}

function state_Change() {
if (xmlhttp.readyState==4) {// 4 = "loaded"
if (xmlhttp.status==200) {// 200 = OK
// ...our code here...
} else {
alert("Problem retrieving XML data");
}
}
}
</script>

In the script above, "onreadystatechange" is an event handler. The value (state_Change) is the name of a function which is triggered when the state of the XMLHttpRequest object changes. States run from 0 (uninitialized) to 4 (complete). Only when the state = 4, we can execute our code.

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