1 var getXmlHttpRequest = function () {
 2     try{
 3         //主流浏览器提供了XMLHttpRequest对象
 4         return new XMLHttpRequest();
 5     }catch(e){
 6         //低版本的IE浏览器没有提供XMLHttpRequest对象，IE6以下
 7         //所以必须使用IE浏览器的特定实现ActiveXObject
 8         return new ActiveXObject("Microsoft.XMLHTTP");
 9     }
10 };
11 var xhr = getXmlHttpRequest();
12 // readyState 0=>初始化 1=>载入 2=>载入完成 3=>解析 4=>完成
13 // console.log(xhr.readyState);  0
14 xhr.open("TYPE", "URL", true);
15 // console.log(xhr.readyState);  1
16 xhr.send();
17 // console.log(xhr.readyState);  1
18 xhr.onreadystatechange = function () {
19     // console.log(xhr.status); //HTTP状态吗
20     // console.log(xhr.readyState);  2 3 4
21     if(xhr.readyState === 4 && xhr.status === 200){
22         alert(xhr.responseText);
23     }
24 };

var getXmlHttpRequest = function () {if (window.XMLHttpRequest) {return new XMLHttpRequest();}else if (window.ActiveXObject) {return new ActiveXObject("Microsoft.XMLHTTP");}
};
var xhr = getXmlHttpRequest();
xhr.open("get", "1.txt", true);
xhr.send();
xhr.onreadystatechange = function () {if (xhr.readyState === 4) {alert(xhr.responseText);}
};

var getXmlHttpRequest = function () {try{return new XMLHttpRequest();}catch(e){return new ActiveXObject("Microsoft.XMLHTTP");}
};
var xhr = getXmlHttpRequest();
xhr.open("get", "1.txt", true);
xhr.send();
xhr.onreadystatechange = function () {if (xhr.status === 200) {alert("readyState=" + xhr.readyState + xhr.responseText);}
};