js 检测远端网站是否可以访问

function ifUp(url,onUp,onDown) {
// make random string
var RANDOM_DIGITS = 7; // this is sufficient. Don't do more than ~12.
var pow = Math.pow(10,RANDOM_DIGITS);
var randStr = String(Math.floor(Math.random()*pow)+pow).substr(1);
// create and load image
var img = new Image();
img.onload = onUp;
img.onerror = onDown;
img.src = url+"?"+randStr;
}

ifUp("http://www.taobao.com/favicon.ico",
function(){
// do something
alert("Server is responding");
},
function(){
// do something else
alert("Server is *not* responding");
}
);