很多时候大家做前端时，需要在DOM树载入时马上执行一些函数，也就是实现document ready，而不是实现window.onload，比如对导航条进行初始化。但又不愿意仅为了这一个需求而引入整个jQuery库，于是就把 jQuery的ready方法提取出来，单独使用了。
代码如下：var isReady=false; //判断onDOMReady方法是否已经被执行过
var readyList= [];//把需要执行的方法先暂存在这个数组里
var timer;//定时器句柄
ready=function(fn) {
if (isReady )
fn.call( document);
else
readyList.push( function() { return fn.call(this);});
return this;
}
var onDOMReady=function(){
for(var i=0;i<readyList.length;i++){
readyList[i].apply(document);
}
readyList = null;
}
var bindReady = function(evt){
if(isReady) return;
isReady=true;
onDOMReady.call(window);
if(document.removeEventListener){
document.removeEventListener("DOMContentLoaded", bindReady, false);
}else if(document.attachEvent){
document.detachEvent("onreadystatechange", bindReady);
if(window == window.top){
clearInterval(timer);
timer = null;
}
}
};
if(document.addEventListener){
document.addEventListener("DOMContentLoaded", bindReady, false);
}else if(document.attachEvent){
document.attachEvent("onreadystatechange", function(){
if((/loaded|complete/).test(document.readyState))
bindReady();
});
if(window == window.top){
timer = setInterval(function(){
try{
isReady||document.documentElement.doScroll('left');//在IE下用能否执行doScroll判断dom是否加载完毕
}catch(e){
return;
}
bindReady();
},5);
}
}
})();
使用方式类下：
或
if(agt.toLowerCase().indexOf("msie 6") != -1)
navInitForIE6();//如果浏览器是IE6，则执行navInitForIE6函数来初始化下拉菜单
});
说明：
当我们想在页面加载之后执行某个函数，肯定会想到onload了，但onload在浏览器看来，就是页面上的东西（img,iframe等资源）全部都加载完毕后才能发生，如果页面内有大的图片的话，会在页面展现后好久时间后才执行。
如果只需要对DOM进行操作，那么这时就没必要等到页面全部加载了。我们需要更快的方法。有DOMContentLoaded事件可以轻松解决，可惜的就是IE没有。
关于JSCRIPT的一个方法有段不起眼的话，当页面DOM未加载完成时，调用doScroll方法时，会产生异常。那么我们反过来用，如果不异 常，那么就是页面DOM加载完毕了！这个函数是Diego Perini在07年就发布了这个名为IEContentLoaded的方法，而且获得了广泛认同，以至于现在许多开源框架都是借鉴这种方法，譬如 JQuery中的ready，在注释里有明确说明是有借鉴Diego Perini的IEContentLoaded方法。所以，对于Mozilla & Opera 浏览器，在dom树载入后有现成的 DOMContentLoaded 事件。
对于Safari 浏览器，有document.onreadystatechange事件，当该事件触发时，如果document.readyState=complete时，可视为dom树已经载入。
对于ie，当在iframe内时，同样有document.onreadystatechange事件，对于ie在非iframe内时，只有不断地 通过能否执行doScroll判断dom是否加载完毕。在本例中每间隔5毫秒尝试去执行 document.documentElement.doScroll('left')
另外要注意一点：在ie8下，貌视非iframe窗口也会有document.onreadystatechange事件
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