性能

function updateUI(){
    var imgs = document.getElementByTagName("img");
    for (var i=0, len = imgs.length; i < len; i++){
         imgs[i].title = document.title + " image " + i;
    }

     var msg = document.getElementById("msg");
     msg.innerHTML = "Update complete";
}


//性能优化 : 将一个函数中会用到的多次的全局对象存储为局部变量总是没错的
function updateUI(){
    var doc = document;
    var imgs = doc.getElementsByTagName("img");
    ......
}