????AJAX = Asynchronous JavaScript and XML (????JavaScript??XML)??
????AJAX????????? ?????????????????????????????????￡???????????????????????????????????????????顣
????AJAX??????к??????????????????????????????????????????????????????????μ????????????????????????????????????????????????????AJAX?????????????????????SEO?????????
????????AJAX????????????HTML/XHTML??CSS??JavaScript/DOM???????
????AJAX????????????????????????漰??????????????????漰????????????????????(new XMLHttpRequest())??????????(open())??????????(send())??????????(responseText)??
????????????
????IE9+?????????????????new XMLHttpRequest()??????????????IE8???????????new ActiveXObject()???????д?????
???????????????????????′?????м????
1 try {
2     xml = new ActiveXObject("Msxml2.XMLHTTP");
3 } catch(e) {
4     try {
5         xml = new ActiveXObject("Microsoft.XMLHTTP");
6     } catch(e1) {
7         xml = new XMLHttpRequest();
8     }
9 }
??????????IE11??????????IE10?????2?дnew ActiveXObject("Msxml2.XMLHTTP")??????????????????????????????????
????var xml = window.ActiveXObject ? new ActiveXObject("Microsoft.XMLHTTP") : new XMLHttpRequest();
???????????
????xml.open('get'?? 'url'?? true/false);
??????????????????string. ?????????????????get/post?????????????get
??????????????????string. ??????????????
???????????????????boolean. ???????????????true????????????
????????????
????xml.send();
???????????????????????xml.send(str);
????????????
????xml.onreadystatechange = function() {
????if (xml.readyState == 4 && xml.status == 200) {
????alert(xml.responseText);
????}
????}
????status??????????????????200??404??200???????????404???δ?????檔
????readyState??5???????????0??1??2??3??4?????????????????????onreadystatechange??
????readyState??5????????????
????0: ????δ?????
????1: ???????????????
????2: ?????????
????3: ????????
????4: ???????????????????
?????????????????????????????????????????????????