现在，我们来研究一下如何解决这个问题，解决方法就是在DOM加载完毕之后就执行程序。
先介绍两个人。一，jquery的作者：John Resig；二，javascript的世界级大师：dean edwards。（大家要记住这两位天才！）
jquery里有专门解决DOM加载的函数$(document).ready()（简写就是$(fn)），非常好用！John Resig在《Pro JavaScript Techniques》里，有这样一个方法处理DOM加载，原理就是通过document&& document.getElementsByTagName &&document.getElementById&& document.body 去判断Dom树是否加载完毕。代码如下：
function domReady( f ) {
// 如果DOM加载完毕，马上执行函数
if ( domReady.done ) return f();
// 假如我们已增加一个函数
if ( domReady.timer ) {
// 把它加入待执行的函数清单中
domReady.ready.push( f );
} else {
// 为页面加载完成绑定一个事件,
// 为防止它最先完成. 使用 addEvent（下面列出）.
addEvent( window, “load”, isDOMReady );
// 初始化待执行的函数的数组
domReady.ready = [ f ];
// 经可能快地检查Dom是否已可用
domReady.timer = setInterval( isDOMReady, 13 );
}
}
// 检查Dom是否已可操作
function isDOMReady() {
// 假如已检查出Dom已可用, 忽略
if ( domReady.done ) return false;
// 检查若干函数和元素是否可用
if ( document && document.getElementsByTagName && document.getElementById && document.body ) {
// 假如可用, 停止检查
clearInterval( domReady.timer );
domReady.timer = null;
// 执行所有等待的函数
for ( var i = 0; i < domReady.ready.length; i++ )
domReady.ready[i]();
// 记录在此已经完成
domReady.ready = null;
domReady.done = true;
}
}
// 由 Dean Edwards 在2005 所编写addEvent/removeEvent,
// 由 Tino Zijdel整理
// http://dean.edwards.name/weblog/2005/10/add-event/
//优点是1.可以在所有浏览器工作；
//2.this指向当前元素；
//3.综合了所有浏览器防止默认行为和阻止事件冒泡的的函数
//缺点就是仅在冒泡阶段工作
function addEvent(element, type, handler) {
// assign each event handler a unique ID
if (!handler.$$guid) handler.$$guid = addEvent.guid++;
// create a hash table of event types for the element
if (!element.events) element.events = {};
// create a hash table of event handlers for each element/event pair
var handlers = element.events[type];
if (!handlers) {
handlers = element.events[type] = {};
// store the existing event handler (if there is one)
if (element["on" + type]) {
handlers[0] = element["on" + type];
}
}
// store the event handler in the hash table
handlers[handler.$$guid] = handler;
// assign a global event handler to do all the work
element["on" + type] = handleEvent;
};
// a counter used to create unique IDs
addEvent.guid = 1;
function removeEvent(element, type, handler) {
// delete the event handler from the hash table
if (element.events && element.events[type]) {
delete element.events[type][handler.$$guid];
}
};
function handleEvent(event) {
var returnValue = true;
// grab the event object (IE uses a global event object)
event = event || fixEvent(window.event);
// get a reference to the hash table of event handlers
var handlers = this.events[event.type];
// execute each event handler
for (var i in handlers) {
this.$$handleEvent = handlers[i];
if (this.$$handleEvent(event) === false) {
returnValue = false;
}
}
return returnValue;
};
function fixEvent(event) {
// add W3C standard event methods
event.preventDefault = fixEvent.preventDefault;
event.stopPropagation = fixEvent.stopPropagation;
return event;
};
fixEvent.preventDefault = function() {
this.returnValue = false;
};
fixEvent.stopPropagation = function() {
this.cancelBubble = true;
};
还有一个估计由几个外国大师合作写的，实现同样功能。
/*
* (c)2006 Jesse Skinner/Dean Edwards/Matthias Miller/John Resig
* Special thanks to Dan Webb's domready.js Prototype extension
* and Simon Willison's addLoadEvent
*
* For more info, see:
* http://www.thefutureoftheweb.com/blog/adddomloadevent
* http://dean.edwards.name/weblog/2006/06/again/
* http://www.vivabit.com/bollocks/2006/06/21/a-dom-ready-extension-for-prototype
* http://simon.incutio.com/archive/2004/05/26/addLoadEvent
*
*
* To use: call addDOMLoadEvent one or more times with functions, ie:
*
* function something() {
* // do something
* }
* addDOMLoadEvent(something);
*
* addDOMLoadEvent(function() {
* // do other stuff
* });
*
*/
addDOMLoadEvent = (function(){
// create event function stack
var load_events = [],
load_timer,
script,
done,
exec,
old_onload,
init = function () {
done = true;
// kill the timer
clearInterval(load_timer);
// execute each function in the stack in the order they were added
while (exec = load_events.shift())
exec();
if (script) script.onreadystatechange = '';
};
return function (func) {
// if the init function was already ran, just run this function now and stop
if (done) return func();
if (!load_events[0]) {
// for Mozilla/Opera9
if (document.addEventListener)
document.addEventListener("DOMContentLoaded", init, false);
// for Internet Explorer
/*@cc_on @*/
/*@if (@_win32)
document.write("<script id=__ie_onload defer src=//0><\/scr"+"ipt>");
script = document.getElementById("__ie_onload");
script.onreadystatechange = function() {
if (this.readyState == "complete")
init(); // call the onload handler
};
/*@end @*/
// for Safari
if (/WebKit/i.test(navigator.userAgent)) { // sniff
load_timer = setInterval(function() {
if (/loaded|complete/.test(document.readyState))
init(); // call the onload handler
}, 10);
}
// for other browsers set the window.onload, but also execute the old window.onload
old_onload = window.onload;
window.onload = function() {
init();
if (old_onload) old_onload();
};
}
load_events.push(func);
}
})();