loadScript 和 addEvent

    function loadScript(url, callback, charset){
        charset = charset || 'utf-8';
        callback = callback ||
        function(){
        };
        var t = document.createElement("script");
        t.type = "text/javascript";
        t.charset = charset;
        t.src = url;
        var _fun = function(){
            if (t.onreadystatechange) {
                t.onreadystatechange = null;
            }
            else {
                t.onload = null;
            }
            _fun = null;  // 此句感觉无效
            document.getElementsByTagName("head")[0].removeChild(t);
            callback();
        };
        if (typeof t.onreadystatechange != "undefined") {
            t.onreadystatechange = function(){
                if (t.readyState == 'complete' || t.readyState == 'loaded') {
                    _fun();
                }
            };
        }
        else {
            t.onload = _fun;
        }
        //此句可能有bug
        //document.getElementsByTagName("head")[0].appendChild(t);
        document.getElementsByTagName("head")[0].insertBefore(t,document.getElementsByTagName("head")[0].firstChild);
    }
    
    function addEventListener(dom, eventName, fn){
        if (dom.addEventListener) {
            dom.addEventListener(eventName, fn, false);
        }
        else if (dom.attachEvent) {
                dom.attachEvent("on" + eventName, fn);
            }
        else
        {
            dom["on" + eventName] = fn;
        }
    }