页面初始化中，用的较多的就是$(document).ready(function(){//代码}); 或 $(window).load(function(){//代码});
他们的区别就是，ready是在DOM的结构加载完后就触发，load是在页面内包括DOM结构，css，js，图片等都加载完成后再触发，显然ready更适合作为页面初始化使用。但有时候也不尽然。需要进一步查看其内部机制。
那么ready的内部是如何判断DOM的结构加载完的？并且不同的浏览器的判断是如何的？
答案就在jquery代码内，假设jquery的版本是jquery-1.11.3.js。
ready的关键代码（3507~3566行），关键代码用红色标出：
jQuery.ready.promise = function( obj ) {
if ( !readyList ) {
readyList = jQuery.Deferred();
// Catch cases where $(document).ready() is called after the browser event has already occurred.
// we once tried to use readyState "interactive" here, but it caused issues like the one
// discovered by ChrisS here: http://bugs.jquery.com/ticket/12282#comment:15
if ( document.readyState === "complete" ) {
// Handle it asynchronously to allow scripts the opportunity to delay ready
setTimeout( jQuery.ready );
// Standards-based browsers support DOMContentLoaded
} else if ( document.addEventListener ) {
// Use the handy event callback
document.addEventListener( "DOMContentLoaded", completed, false );
// A fallback to window.onload, that will always work
window.addEventListener( "load", completed, false );
// If IE event model is used
} else {
// Ensure firing before onload, maybe late but safe also for iframes
document.attachEvent( "onreadystatechange", completed );
// A fallback to window.onload, that will always work
window.attachEvent( "onload", completed );
// If IE and not a frame
// continually check to see if the document is ready
var top = false;
try {
top = window.frameElement == null && document.documentElement;
} catch(e) {}
if ( top && top.doScroll ) {
(function doScrollCheck() {
if ( !jQuery.isReady ) {
try {
// Use the trick by Diego Perini
// http://javascript.nwbox.com/IEContentLoaded/
top.doScroll("left");
} catch(e) {
return setTimeout( doScrollCheck, 50 );
}
// detach all dom ready events
detach();
// and execute any waiting functions
jQuery.ready();
}
})();
}
}
}
return readyList.promise( obj );
};
上面的代码在触发ready时可以分成两部分
1.标准浏览器下的触发
当浏览器是基于标准浏览器时，会在加载完DOM结构后触发“DOMContentLoaded”事件，jquery内部就用此事件作为ready的触发源。
document.addEventListener( "DOMContentLoaded", completed, false );
2.IE浏览器下的触发
当浏览器是IE浏览器时，因为IE浏览器（蛋疼并强大着）不支持“DOMContentLoaded”事件，所以只能另谋它法，
(function doScrollCheck() {
if ( !jQuery.isReady ) {
try {
// Use the trick by Diego Perini
// http://javascript.nwbox.com/IEContentLoaded/
top.doScroll("left");
} catch(e) {
return setTimeout( doScrollCheck, 50 );
}
// detach all dom ready events
detach();
// and execute any waiting functions
jQuery.ready();
}
})();
IE下的做法 就是上面代码的红字处，用“document.documentElement.doScroll("left")”的方法去滚动页面，如果没加载完就等个50毫秒后继续滚，直到滚的动后就触发ready。
但是，如果页面中有frame的场合，会使用window.onload事件作为ready的触发源。
所以在IE下，页面中有frame时，ready也是等到页面内的所有内容加载完成后再触发。