//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
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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<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>