Replace the jQuery Document Ready Function with JavaScript
jQuery's $(document).ready() method is used to execute code after the DOM is fully loaded. Since it executes the given function when all DOM elements are available, it ensures that attempts to access or operate on the element work properly.
Before jQuery 3.0, the typical usage of using anonymous functions is as follows:
$(document).ready(function() {
// .ready() 的处理程序被调用。
});Key Points
- jQuery's
document.readymethod ensures that the code is executed only when all DOM elements are safe and actionable, but has been significantly changed in jQuery 3.0 and all syntax methods (except$(handler);) are deprecated. DOMContentLoadedEvents are pure JavaScript alternatives tojQuery readymethods that can be used in modern browsers and IE9. For older versions of IE, theonreadystatechangeevent can be used.- In many cases, any of these solutions may not be required. Just put the JavaScript code before the end tag
</body>to make sure the DOM is loading.
Changes to ready() in jQuery 3.0
Before release of version 3.0, you can call the ready method in several ways:
- On the document element:
$(document).ready(handler); - On the empty element:
$().ready(handler); - or call directly (i.e. not on a specific element):
$(handler);
All the above variants are functionally equivalent. Regardless of which element is called, the specified handler will be called as long as the DOM is fully loaded. In other words, calling it on an image element $("img") is no different from calling it on a document element, and the trigger time of the callback function has nothing to do with the loading of the specified element. Instead, it will be called after the entire DOM is fully loaded.
In jQuery 3.0, all other syntax methods except $(handler); are deprecated. The official reason is:
This is because the choice has nothing to do with the behavior of the
.ready()method, which is inefficient and may lead to false assumptions about the behavior of the method.
ready Difference between an event and a load
Events are fired when the DOM is fully loaded and elements are safely accessible. On the other hand, the ready event fires after the DOM and all resources have been loaded. load
Events can be used as follows: load
$(window).on("load", function(){
// 所有资源（包括图像）加载完毕时的处理程序
});For normal DOM operations, you may not need the
event, but if you want to show a load spinner before all resources are loaded, or if you want to do some calculations with image size, it might be the right choice . load
You may not need jQuery.ready()
Theready method ensures that the code is executed only when all DOM elements are safe and actionable. But what does this mean? When you execute JavaScript code in the <body> part of the HTML document, this will ensure that the code is executed after the browser also loads all subsequent elements (for example, <code><p> elements):
$(document).ready(function() {
// .ready() 的处理程序被调用。
});If you execute JavaScript code as the last thing in <body>, you probably don't need to wrap it in ready() because all the elements you may try to operate or access are loaded:
$(window).on("load", function(){
// 所有资源（包括图像）加载完毕时的处理程序
});Pure JavaScript ready() Alternatives
For modern browsers and IE9, you can listen to DOMContentLoaded events:
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>.ready() 教程</title> <🎜> </head> <body> <🎜> <p>我是这个网站的内容</p> </body> </html>
However, note that if the event has been fired, the callback function will not be executed. To ensure that the callback function is always run, jQuery checks the document's readyState (reference) and executes the callback function immediately if it has been completed:
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>.ready() 教程</title> </head> <body> <p>我是这个网站的内容</p> <🎜> <🎜> </body> </html>
You can also include the domReady library, which already implements this solution.
Old version of Internet Explorer
For IE versions less than or equal to 8, you can use the onreadystatechange event to detect the document's readyState:
document.addEventListener("DOMContentLoaded", function(){
// DOM 完全加载时的处理程序
}); Or, you can use the load event like jQuery, as this will work in any browser. This also causes time delays, as it will wait for all resources to load. Note that you also have to check readyState as explained above to ensure that the callback function will still be executed even if the event has been fired.
Conclusion
If you are looking for a pure JavaScript alternative to the ready method, you can use the DOMContentLoaded event. If your system requirements include IE, you may need to use onreadystatechange events or load events.
If you are using jQuery in your project, you can safely continue to use jQuery's document.ready function, but remember to avoid using (deprecated) ready() methods on elements as mentioned earlier ( For example $(document).ready()).
Lastly, don't forget that in many cases you may not need any of these solutions - just move your JavaScript code to the end tag
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