Ajax Flow
1)Create a XMLHttpRequest object from browser.
2)Send a HttpRequest from browser.
3)Server process a HttpRequest.
4)Server create a response and send data back to the browser.
5)Browser process the server response returned data and update to the page content.

Property	Description
responseText	Get the response data as a string. Read-only. document.getElementById("divEmployee").innerHTML=xmlhttp.responseText;
responseXML	Get the response data as XML. Read-only. var xmldoc = xmlhttp.responseXML; var empList = ""; var emp = xmlDoc.getElementsByTagName("Employee"); for (i=0;i<emp.length;i++) {   empList = empList + emp[i].childNodes[0].nodeValue; } document.getElementById("divEmpList").innerHTML = empList;
onreadystatechange	Contains the name of the event handler function to be called when the value of the readyState property changes. Read/write. The onreadystatechange event is triggered four times, one time for each change in readyState.
readyState	Contains state of the XMLHttpRequest. Read-only. 0: Un initialized 1: server connection established - Loading 2: request received - Loaded 3: processing request - Interactive 4: request finished and response is ready - Complete
status	Contains the HTTP status code returned by a request. Read-only. 200 OK 201 Created 204 No Content 205 Reset Content 206 Partial Content 400 Bad Request 401 Unauthorized 403 Forbidden 404 Not Found 405 Method Not Allowed 406 Not Acceptable 407 Proxy Authentication Required 408 Request Timeout 411 Length Required 413 Requested Entity Too Large 414 Requested URL Too Long 415 Unsupported Media Type 500 Internal Server Error 501 Not Implemented 502 Bad Gateway 503 Service Unavailable 504 Gateway Timeout 505 HTTP Version Not Supported
statusText	Contains the HTTP response status text. Read-only. "OK" - 200 "Page not found" - 404

Method	Description
setRequestHeader(headerName, headerValue)	Sets the name and value of an HTTP header.
abort	Stops the HTTP request and resets its readyState back to zero.
getAllResponseHeaders	Returns all the HTTP headers.
getResponseHeader(headerName)	Returns the value of an HTTP header.
open(method, url, asynchronous flag, username, password)	Opens a request to the server.
send(string)	Sends an HTTP request to the server.

It is used to communicate with the server.


To send a request to a Server, use the open() and send() methods.

open(method, URL/FILE [,async] [,username][,password])
Items in [] are optional.
method - used to open the connections, the type of request: GET, POST or HEAD.
URL/FILE - requested url(like asp,php,jsp or servlet) or file(like txt, xml) on the server.
async - a boolean value, true (asynchronous) or false (synchronous), default is true.
username - username of your account.
password - password of your account.

send(string) - string is used for only POST requests.



It is simpler and faster than POST, and can be used in most cases.
POST method requests POST method is more robust and secure than GET method.
POST method has no size limitations.
Sending user input.
Sending a large amount of data to the server.
A cached file is not an option (update a file or database on the server).



The function to execute when the response is ready in the onreadystatechange event

The function will not continue to execute, until the server response is ready.
If the server is busy or slow, the application will hang or stop.
When you use asynchronousFlag is false, do not write an onreadystatechange function, just put the code after the send() statement.