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How to replace JS with jQuery's Read() method

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Release： 2018-04-23 17:10:06

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This time I will show you how jQuery's Read() method replaces JS, and jQuery's Read() method replaces JSWhat are the precautions?The following is a practical case, let's take a look.

As we all know, the ready method in jQuery executes the code in the DOM immediately after it is completely downloaded. Because it waits for all DOM elements to be loaded before executing the given function, you can be sure that methods that attempt to operate and access element nodes will be executed. This article will introduce to you how to use jQuery's Read() method instead of native JS. Let's take a look.

Before jQuery 3.0, the classic usage of ready was to use an anonymous function, like this:

$(document).ready(function() {
 // Handler for .ready() called.
});

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jQuery 3.0 ready() changes

Before jQuery 3.0 was released, there were several methods called ready methods:

Operating on the document element: $(document).ready (handler);

Operate on empty elements: $().ready(handler);

Or directly (that is, not on a specific element ) Operation: $(handler);

All named variants above are functionally equivalent. No matter which element it is, its specified handler will be called after the DOM is loaded. In other words, the completion of DOM loading here does not mean that a specific element in the document, such as the img element, has been loaded. On the contrary, what it means here is that the entire DOM tree has been loaded.

In jQuery 3.0, all ready methods except $(handler) are deprecated.

Official statement:

This is because the selector is not connected to ready(), which is not only inefficient but also causes the browser engine to block the method. Behavior makes incorrect assumptions.

The difference between the ready event and the load event

The ready event is triggered when the DOM is loaded and the element can be safely accessed. On the other hand, the load event is triggered after the DOM and all resources are loaded.

You can use the load event as follows:

$(window).on("load", function(){
 // Handler when all assets (including images) are loaded
});

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In this case, not only do you have to wait until the DOM structure can be fully accessed, but you also need to wait until all image resources are completely loaded (the loading time depends on image file size) to execute the function.

You may not need the load event for normal DOM operations, but if you want to display a rotating loader style before all resources are loaded, for example, or you want to use JS to calculate the image size, this might be a good choice.

You may not need jQuery.ready()

The ready method ensures that code is only executed when all DOM elements can be safely manipulated . But what does this mean? This means that when the js code you want to execute is embedded in a certain fragment in HTML, the browser must also load the following elements before it can be executed.

Like the following example:

<!doctype html>
<html>
 <head>
 <meta charset="utf-8">
 <title>.ready() tutorial</title>
 <script src="https://cdn.jsdelivr.net/jquery/latest/jquery.min.js"></script>
 <script>
  $(function(){ // .ready() callback, is only executed when the DOM is fully loaded
  var length = $("p").length;
  // The following will log 1 to the console, as the paragraph exists.
  // This is the evidence that this method is only called when the
  // DOM is fully loaded
  console.log(length);
  });
 </script>
 </head>
 <body>
 <p>I'm the content of this website</p>
 </body>
</html>

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If the javascript code you want to execute is placed at the end of the body, you may not need to use ready() Method, because the DOM element you may be trying to manipulate and access when the browser parses the javascript has already been loaded:

<!doctype html>
<html>
 <head>
 <meta charset="utf-8">
 <title>.ready() tutorial</title>
 </head>
 <body>
 <p>I'm the content of this website</p>
 <script src="https://cdn.jsdelivr.net/jquery/latest/jquery.min.js"></script>
 <script>
  var length = $("p").length;
  // The following will log 1 to the console, as the paragraph exists.
  console.log(length);
 </script>
 </body>
</html>

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Native JavaScript ready() replacement

For modern browsers and IE9, you can achieve the same functionality of ready() by listening to the DOMContentLoaded event:

document.addEventListener("DOMContentLoaded", function(){
 // Handler when the DOM is fully loaded
});

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However, please note that if the event has already been emitted , the callback will not be executed. To ensure that the callback always runs, jQuery checks the "readyState" attribute of the document reference), and if the attribute value changes to complete, the callback function is executed immediately:

var callback = function(){
 // Handler when the DOM is fully loaded
};
if (
 document.readyState === "complete" ||
 (document.readyState !== "loading" && !document.documentElement.doScroll)
) {
 callback();
} else {
 document.addEventListener("DOMContentLoaded", callback);
}

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Includes the domReady library, which has been implemented Got this solution.

Old versionIE browser

For browsers IE8 and below, you can use the onreadystatechange event to Monitor the readyState attribute of the document:

document.attachEvent("onreadystatechange", function(){
 // check if the DOM is fully loaded
 if(document.readyState === "complete"){
 // remove the listener, to make sure it isn't fired in future
 document.detachEvent("onreadystatechange", arguments.callee);
 // The actual handler...
 }
});

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或者你可以使用Load事件，如jQuery，这样可以在任何浏览器上运行。这也会导致一个时间延迟，因为它会等待所有的资产被加载。

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