Detailed explanation of the differences between document and window and load and ready in jQuery_jquery

Children who have used JavaScript should know the window object and document object, and should also have heard of the load event and ready event. Of course Xiaocai also knows it, and thinks he understands it very well. It was not until something went wrong recently that he realized that it was not the case. So simple.

First of all, let’s talk about window and document. Intuitively speaking, window represents the browser window, and document represents the dom element loaded in the browser window. Furthermore, document is an attribute of window, and window It is the top-level object.

What’s the difference between the two? It's easy to understand. Suppose there is a browser now, and the page loaded in it is very long and exceeds one screen. Of course, scroll bars will definitely appear. At this time, $(window).height() and $(document). height() is not equal, the height of the document must be larger than that of the window, because the window is always that big. See the picture below:

Let’s talk about the load event and ready event (load and ready here refer to jQuery events, the same below).

Let’s talk about load first. The load event is mainly used to replace the native window.onload. It can only be used in two scenarios:

·On the window object. For example $(window).load(fn);.

· Elements with URLs (images, scripts, frames, iframes). For example $("img").load(fn);.

In addition, there is no load event for any element, such as: $(document).load(fn); this is wrong writing and will not be executed at all.

The load event needs to be completely loaded before the page can be triggered. The so-called complete loading means not only that the DOM structure is loaded, but also that all link references are loaded. For example, if there are a large number of pictures on the page, you must wait until each picture is loaded before it is fully loaded.

The most important thing is that the jQuery official document clearly states that the cross-browser compatibility of the load event is very poor (It doesn't work consistently nor reliably cross-browser). After Xiaocai testing, Google Chrome only supports $(window).load(fn);, while Firefox supports $(window).load(fn); and $("img").load(fn);.

Therefore, it is strongly not recommended to use the load event unless necessary.

Finally, let’s talk about ready. The ready event can be added to any element, such as $(window).ready(fn);, $(document).ready(fn);, $(“div”).ready(fn );etc.

The ready event does not require the page to be completely loaded, it only needs to be loaded to trigger the DOM structure.

Multiple ready events can be registered at the same time. When executed, they will be executed in the order of registration. Note that even if you register the ready events of different elements, they will be executed in order. For example, the following code:

According to common sense, the div should be loaded first, so alert("div"); is executed first, and then alert("document"); or alert("window");, but unfortunately, alert( "div"); is the last one executed. Therefore, no matter whether the ready event is registered on the same element or not, it will be executed in the order of registration.

The last item, the ready event conflicts with window.onload (or ). If window.onload (or ) is used, it will cause ready event is not executed.

After so much discussion, it was finally proven that: $(document).ready(fn); has the best compatibility and security. If there is such a need, try to use this method.