This document provides an overview of AJAX (Asynchronous JavaScript and XML). It discusses how AJAX allows a browser to communicate with a server asynchronously by making HTTP requests without reloading the entire page. This is done using the XMLHttpRequest object, which can retrieve data from the server in the background and update parts of the page without reloading. The document provides examples of how to create an XMLHttpRequest object, make GET requests, and handle responses. It also summarizes key properties and methods of the XMLHttpRequest API.

1. AJAX The basics

2. WEB
•Client - Server
Web ServerWeb Browser SQL Server
TCP/IPInternet (HTTP)
Simplified Diagram

3. HTTP
•Stateless protocol (ignore session and viewstate)
•Serves entire state (page?) to browser every time
•Postback ENTIRE PAGE to page form (submit button)
•How can the browser communicate with server without Postback?

4. AJAX?
•Microsoft invented XMLHttpRequest Objects
•ActiveX object before IE7 ActiveXObject("Msxml2.XMLHTTP")
•XMLHttpRequest Calls should be asynchronous
•Asynchronous Javascript And XML

5. AJAX
•Requests cannot be cross domain (ignore CORS)
•Browser can make requests without reloading entire page (client side)
•Talks to server through XHR (XMLHttpRequest)
•XHR can pass more than just XML

6. CHEAT SHEET
•Create XHR Object
•Call xhr.Open pass “calling method”, url, async (true, false),
username, pw
•Check readyState – returned 4 means returned with data

7. AJAX
•Create wrapper function
function createXMLHttpRequest() {
try {
return new XMLHttpRequest();
} catch(e) {}
try {
return new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {}
alert("XMLHttpRequest not supported");
return null;
}

8. AJAX
•Call createXMLHttpRequest
var xhReq = createXMLHttpRequest();
xhReq.open("GET", url, true);
xhReq.onreadystatechange =
function {
if (xhReq.readyState != 4) { return; }
var serverResponse = xhReq.responseText;
//var serverResponse = xhReq.responseXML;
}
xhReq.send(null);

9. API SUMMARY
•XMLHttpRequest Object Properties
•onreadystatechange: Callback function
•readyState: State of the request cycle
•responseText: Text response from the server
•responseXML: XML response from the server
•status: HTTP response code received from the server
•statusText: HTTP response code description received from the server

10. API SUMMARY
•XMLHttpRequest Object Methods
•abort(): Stops the request and resets its readyState back to zero.
•getAllResponseHeaders(): Returns a string of all response headers, separated by a newline as in
the original message.
•getResponseHeader(headerField): Returns the value for a particular header field.
•open(requestMethod, url, asynchronousFlag, username, password): Prepares XMLHttpRequest.
Only the first two parameters are required. username and password can be used for
authentication.
•send(bodyContent): Sends the message along with specified body content (null if no body
content is to be sent, e.g. for GET requests).
•setRequestHeader(headerField, headerValue): Sets a request header.