$(document).ready()和window.onload的区别

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The window.onload event fires when a document is completely downloaded to the
browser. This means that every element on the page is ready to be manipulated by
JavaScript, which is a boon for writing feature-rich code without worrying about
load order.
On the other hand, a handler registered using $(document).ready() is invoked
when the DOM is completely ready for use. This also means that all elements are
accessible by our scripts, but does not mean that every associated file has been
downloaded. As soon as the HTML file has been downloaded and parsed into a
DOM tree, the code can run.

Consider, for example, a page that presents an image gallery; such a page may have
many large images on it, which we can hide, show, move, and otherwise manipulate
with jQuery. If we set up our interface using the onload event, users will have to
wait until each and every image is completely downloaded before they can use those
features. Even worse, if behaviors are not yet attached to elements that have default
behaviors (such as links), user interactions could produce unintended outcomes.
However, when we use $(document).ready() for the setup, the interface is ready
to be used earlier with the correct behavior.