不同浏览器创建XMLHttpRequest对象
function getXmlHttpRequest(){
var xmlHttpRequest;
try{
xmlHttpRequest = new XMLHttpRequest();
}catch(e){
try{
// Internet Explorer
xmlHttpRequest = new ActiveXObject("Msxml2.XMLHTTp");
}catch(e){
try{
// Internet Explorer
xmlHttpRequest = new ActiveXObject("Microsoft".XMLHTTP);
}catch(e){
}
}
}
return xmlHttpRequest;
}