ajax浏览器兼容

 http://bolm.cn/tutorial/ajax_xmlhttp/ajax_index.html
http://www.ad0.cn/netfetch/go.php/category/25/
http://www.ad0.cn/netfetch/go.php/category/16/
http://www.ad0.cn/netfetch/read.php/976.htm

//方便
function createXmlHttp()
{
    try{
        if(window.ActiveXObject){
            for(var i=7;i;i--){
                try{
                    if(i==2){
                        xmlhttp_request = new ActiveXObject( "Microsoft.XMLHTTP" );
                    }else{
                        xmlhttp_request = new ActiveXObject( "Msxml2.XMLHTTP." + i + ".0" );
                    }
                    xmlhttp_request.setRequestHeader("Content-Type","text/xml");
                    xmlhttp_request.setRequestHeader("Content-Type","gb2312");
                    break;
                }
                catch(e){
                    xmlhttp_request = false;
                }
            }
        }else if(window.XMLHttpRequest ){
            xmlhttp_request = new XMLHttpRequest();
            if (xmlhttp_request.overrideMimeType) {
                xmlhttp_request.overrideMimeType('text/xml');
            }
        }
    }catch(e){
        xmlhttp_request = false;
    }
    return xmlhttp_request;
}

//安全
var xmlhttp=null;

if(window.XMLHttpRequest)
{
         xmlhttp=new XMLHttpRequest();
}
if(!xmlhttp&&window.ActiveXObject)
{
 
 try
 {
     xmlhttp=new ActiveXObject("Msxml2.XMLHTTP.5.0")
 }
 catch(e)
 {
  try
  {
        xmlhttp=new ActiveXObject("Msxml2.XMLHTTP.4.0")
  }
  catch(e){
   try
   {
    new ActiveXObject("Msxml2.XMLHTTP")
   }
   catch(e)
   {
    try{
     new ActiveXObject("Microsoft.XMLHTTP")
    }catch(e){
 
    }
   
   
   }
  
  
  }
 
 
 }
 
 
}

//不错
function createXMLHttp() {
var aVersions = [ "MSXML2.XMLHttp.5.0",
"MSXML2.XMLHttp.4.0","MSXML2.XMLHttp.3.0",
"MSXML2.XMLHttp","Microsoft.XMLHttp"
];
for (var i = 0; i < aVersions.length; i++) {
try {
var oXmlHttp = new ActiveXObject(aVersions[i]);
return oXmlHttp;
} catch (oError) {
//Do nothing
}
}
throw new Error("未安装MSXML！");
}

function createXMLHttp() {
if (typeof XMLHttpRequest != "undefined") {
return new XMLHttpRequest();
} else if (window.ActiveXObject) {
var aVersions = [ "MSXML2.XMLHttp.5.0",
"MSXML2.XMLHttp.4.0","MSXML2.XMLHttp.3.0",
"MSXML2.XMLHttp","Microsoft.XMLHttp"
];
for (var i = 0; i < aVersions.length; i++) {
try {
var oXmlHttp = new ActiveXObject(aVersions[i]);
return oXmlHttp;
} catch (oError) {
//Do nothing
}
}
}
throw new Error("不能创建XMLHttp对象！");
}