关于XML HTTP 的疑问 - Web 开发 / JavaScript

JScript code:
//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{{