核心思路：
标准浏览器(含IE9+)比较简单，直接监听DOMContentLoaded事件；
低版本的IE(IE678)两套机制:
1)尝试轮询document.documentElement.doScroll("left")是否报错，若报错则dom树结构未ready，否则dom ready；
2)监听document的onreadystatechange事件，判断document.readyState是否为"complete"。
参考：http://javascript.nwbox.com/IEContentLoaded/
直接上源码了:
var domReady = (function () {
var doc = document,
branch = document.addEventListener ? 'w3c' : 'ie678';
var _domReady = {
// When _domReady.done is true，all 'fn' will be invoked immediately.
done: false,
// The stack which all functions will be pushed into
fn: [],
// push callback functions
push: function (fn) {
if (!_domReady.done) {
// only bind once
if (_domReady.fn.length === 0) {
_domReady.bind();
}
_domReady.fn.push(fn);
} else {
fn();
}
},
// The Real DOMContentLoaded Callback Function
ready: function () {
// Flag DOMContentLoaded Event was Done over
_domReady.done = true;
var fn = _domReady.fn;
for (var i = 0, l = fn.length; i < l; i++) {
fn[i]();
}
_domReady.unbind();
_domReady.fn = null;
},
bind: {
w3c: function () {
doc.addEventListener('DOMContentLoaded', _domReady.ready, false);
},
//IE的监听
ie678: function () {
var done = false,
// only fire once
init = function () {
if (!done) {
done = true;
_domReady.ready();
}
};
// polling for no errors
(function () {
try {
// throws errors until after ondocumentready
doc.documentElement.doScroll('left');
} catch (e) {
setTimeout(arguments.callee, 20);
return;
}
// no errors, fire
init();
})();
// trying to always fire before onload
doc.onreadystatechange = function () {
if (doc.readyState == 'complete') {
doc.onreadystatechange = null;
init();
}
};
}
}[branch],
unbind: {
w3c: function () {
doc.removeEventListener('DOMContentLoaded', _domReady.ready, false);
},
ie678: function () { /* Nothing to do */
}
}[branch]
};
return _domReady.push;
})();使用方法：
可以尝试跟window.onload做个比较
window.onload = function(){
alert("onload");
};
domReady(function(){
alert("domReady 1");
});
domReady(function(){
alert("domReady 2");
});