ajax is an async transferring information way.
three ways to exchange data in today.
1. use XMLHttpRequest,
normal way .but some attention. first ,xhr.readystate==3 means loading the message which is said streaming but ie not support it well.
also, for post and get.while send message to server not get something from it and you url is no more than 2048 characters,just use get method.
for the get method also cache info.other situation,use post method.
2 use script tag.
use this way can cross domain to get js.
var script=document.createElement(‘script‘);
script.src=‘a.js‘;
document.getelementbytagname(‘head‘)[0].appendChild(‘script‘);
function callback(jsonstring){
var data=eval(jsonstring);
}
for this method,a.js must be executable js code.
for a.js ,it‘s callback({‘a‘:1,"b":32});
3 mxhr not very clear other time for search
send message.
two way .
one normal ajax get or post method.the other is beacons
var url=‘index.php‘;
var para=[‘stp=1‘,‘sdd=2‘];
(new Image()).src=url+‘?‘.para.join(‘&‘);
by the way, server will not send anything to customer side.
ajax profience.
use header in server side to last the expire day of request so ajax will not send other request to server and just use the cache.
all server script can set header .for php, it‘s header(‘Expires:‘.gmdate(‘D,d m y h:i:s‘).time()+$lifetime.‘GMT‘);
原文：http://www.cnblogs.com/wz0107/p/4954466.html