JS创建Ajax的XMLHttpRequest对象的通用方法

 function createXMLHttpRequest() {
     var request = false;
     if(window.XMLHttpRequest) {
         request = new XMLHttpRequest();
         if(request.overrideMimeType) {
             request.overrideMimeType('text/xml');
         }
     } else if(window.ActiveXObject) {
         var versions = ['Microsoft.XMLHTTP', 'MSXML.XMLHTTP', 'Microsoft.XMLHTTP', 'Msxml2.XMLHTTP.7.0', 'Msxml2.XMLHTTP.6.0', 'Msxml2.XMLHTTP.5.0', 'Msxml2.XMLHTTP.4.0', 'MSXML2.XMLHTTP.3.0', 'MSXML2.XMLHTTP'];
         for(var i=0; i<versions.length; i++) {
             try {
                 request = new ActiveXObject(versions[i]);
                 if(request) {
                     return request;
                 }
             } catch(e) {}
         }
     }
     return request;
 }