关于XML HTTP 的疑问

hibin101 2010-05-31 11:26:38

//XML HTTP请求对象的创建方法

 if(typeof XMLHttpRequest == "undefined" && window.ActiveXObject){

   function XMLHttpRequest(){

     var arrSignatures = ["MSXML2.0.XMLHTTP.5.0","MSXML2.0.XMLHTTP.4.0","MSXML2.0.XMLHTTP.3.0",

	                      "MSXML2.0.XMLHTTP.2.0","MSXML2.0.XMLHTTP","Microsoft.XMLHTTP"];

     for(var i=0;i < arrSignatures.length;i++){

	    try{

		  var oRequest = new ActiveXObject(arrSignatures[i]);

		  return oRequest;

		}catch(oError){

		  //ignore

		}

	 }

	 throw new Error("MSXML is not installed on your system.");

   }

 }



 var oRequest = new XMLHttpRequest();

 oRequest.open("get","example.txt",false);

 oRequest.send(null);

 alert("status is"+oRequest.status+"("+oRequest.statusText+")");

 alert("Response text is"+oRequest.responseText);

上面这段代码，在IE中是如何执行的呢？当new XMLHttpRequest()时找不到XMLHttpRequest对象时就会去执行上面的if语句块吗？javascript不是从上到下解释执行的吗？怎么会执行下面的再执行上面的呢，不明白，请教教我

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toury 2010-05-31

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你别一叶障目不见泰山啊；一个if遮住了你的双眼，呵呵。关键是function XMLHttpRequest()



<html>

<body>

<script>

if(typeof XMLHttpRequest == "undefined" && window.ActiveXObject){

   function XMLHttpRequest(){

   alert(2)

     var arrSignatures = ["MSXML2.0.XMLHTTP.5.0","MSXML2.0.XMLHTTP.4.0","MSXML2.0.XMLHTTP.3.0",

                          "MSXML2.0.XMLHTTP.2.0","MSXML2.0.XMLHTTP","Microsoft.XMLHTTP"];

     for(var i=0;i < arrSignatures.length;i++){

        try{

          var oRequest = new ActiveXObject(arrSignatures[i]);

          alert(3)

          return oRequest;

        }catch(oError){

          //ignore

        }

     }

     throw new Error("MSXML is not installed on your system.");

   }

 }

alert(1)

 var oRequest = new XMLHttpRequest();

 alert('开始~~')

 oRequest.open("get","example.txt",false);

 oRequest.send(null);

 alert("status is"+oRequest.status+"("+oRequest.statusText+")");

 alert("Response text is"+oRequest.responseText);   

</Script>

</body>

</html>