XMLHttpRequest

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// 封装xhr对象，兼容各个版本
       function createXHR() {
           // 判断游览器是否将XMLHttpRequest作为本地对象实现，针对IE7，Firefox。opera等
           if (typeof XMLHttpRequest != "undefined") {
               return new XMLHttpRequest();
           } else if (typeof ActiveXObject != "undefined") {
               // 将所有可能出现的ActiveXObject版本放在一个数组中
               var xhrArr = ["Microsoft.XMLHTTP", "MSXML2.XMLHTTP.6.0", "MSXML2.XMLHTTP.5.0", "MSXML2.XMLHTTP.4.0",
                   "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP.2.0"
               ];
               var xhr=null;
               // 遍历创建XMLHttpRequest对象
               var len=xhrArr.length;
               for(var i=0;i<len;i++) {
                   try{
                       xhr= new ActiveXObject(xhrArr[i]);
                       break;
                   }
                   catch(ex) {
                       throw("ActiveXObject版本不支持");
                   }
               }
               return xhr;
           }else {
               throw  new Error("No XHR object availabel.")
           }
       }

       var xhr=createXHR();
       console.log(xhr);

简易版本

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var xhr;
if(window.XMLHttpRequest) {
    xhr=new XMLHttpRequest();
}else {
    xhr=new ActiveXObject("Microsoft.XMLHTTP")
}