script src 指定一个无效路径(跨域)

var _script = document.createElement("script");
_script.onload = _script.onreadystatechange = function(){
　　if(!this.readyState || this.readyState=='loaded' || this.readyState=='complete'){
　　　　_func(true);
　　　　_script.onload = _script.onreadystatechange = null;
　　}else{
_func(false);
}
};
document.getElementsByTagName("head")[0].appendChild(_script);
function _func(F){
if (F){
//do something
}else{
alert("网络异常！");
}
}
script.src = "https://www.jd.com/index.html";//这个网址被屏蔽了，无法访问

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为什么总是能调用到 _func(true);

http://www.jb51.net/article/57273.htm