AJAX: The XMLHttpRequest Object

*Creating Object
For Safari and Mozilla, a simple call to the object's constructor function does the job:
var req = new XMLHttpRequest();
For IE or other browsers use ActiveX, pass the name of the object to the ActiveX constuctor:
var req = new ActiveXObject("Microsoft.XMLHttp");
* Object methods:
The methods supported by Safari 1.2, Mozilla, and IE5 or later:

• abort(): Stops the current request

• getAllResponseHeaders(): Returns complete set of headers (labels and values) as a string.

• getResponseHeader("headerLabel"): Returns the string value of a single header label.

• open("method", "URL"[, asyncFlag[, "userName"[, "password"]]]): Assigns destination URL, method, and other optional attributes of a pending request.

• send(content): Transmits the request, optionally with postable string or DOM object data.

• setRequestHeader("label", "value"): Assigns a label/value pair to the header to be sent with a request.

var req;

function loadAjax(url) {
req = false;
if(window.XMLHttpRequest && !(window.ActiveXObject)) {
try {
req = new XMLHttpRequest();
} catch(e) {
req = false;
}
} else if(window.ActiveXObject) {
try {
req = new ActiveXObject("Msxml2.XMLHTTP");
} catch(e) {
try {
  req = new ActiveXObject("Microsoft.XMLHTTP");
} catch(e) {
  req = false;
}
}
}
if(req) {
req.onreadystatechange = processAjaxFuntion;
req.open("GET", url, true);
req.send("");
}
}

* Object properties
All properties are read-only.

• onreadystatechange: Event handler for an event that fires at every state change.

• readyState: Object status integer:

0 = uninitialized
1 = loading
2 = loaded
3 = interactive
4 = complete

• responseText: String version of data returned from server process.

• responseXML: DOM-compatible document object of data returned from server process.

• status: Numeric code returned by server, such as 404 for "Not Found" or 200 for "OK".

• statusText: String message accompanying the status code.

function processAjaxFuntion() {
// only if req shows "loaded"
if (req.readyState == 4) {
 // only if "OK"
 if (req.status == 200) {
     // ...processing statements go here...
 } else {
     alert("There was a problem retrieving the data:\n" +
         req.statusText);
 }
}
}