创建ajax请求的公用方法

function createReq(){
    //IE 
    if (window.Active){
          var versions = ["Microsoft.XMLHTTP","MSXML2.XMLHttp.6.0",
                                "MSXML2.XMLHttp.5.0","MSXML2.XMLHttp.4.0",
                                "MSXML2.XMLHttp.3.0","MSXML2.XMLHttp.2.0",
                                "MSXML2.XMLHttp"];
          try{
                for ( var i = 0; i < versions.length; i ++) {
                      var oxmlHttp = new ActiveXObject(version[i]);
                      return oxmlHttp;
                }
          }catch(oError){
               
          }
          throw new Error("can't create XMLHttpRequest!");
    }
   // not ie
    if (typeof XMLHTTPRequest != "undefined") {
        var oxmlHttp = new XMLHTTPRequest();
        return oxmlHttp;
   }
}