Summary of methods for asynchronous loading of js files (with code)

This article brings you a summary of the asynchronous loading method of js files (with code). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Why asynchronous loading: Synchronous loading will block the subsequent processing of the browser, that is, the next file loading (such as images), rendering, and code can only be carried out after the current file is loaded. implement. If there are behaviors such as outputting document content, modifying DOM, redirecting, etc. in js, it will cause page blocking.

What is asynchronous loading: Asynchronous loading does not block subsequent operations of the browser, that is, while loading and executing js, other files are loaded in parallel to render the page.

Method 1: async attribute
The async attribute specifies that the js file will be executed as soon as it is available, and the script will be executed asynchronously relative to the rest of the page (when the page continues to parse, the script will be executed)

The async attribute only applies to external scripts (only when using the src attribute).

The async attribute does not guarantee execution order.

<script type="text/javascript" src="demo_async.js" async="async"></script>

Method 2: defer attribute
defer attribute specifies whether to delay script execution until the page is loaded.

The defer attribute indicates that the script will not change the content of the document. The browser knows that it will be able to safely read the remainder of the document without executing the script. It will defer interpretation of the script until the document has been displayed to the user. until. to speed up document processing.

Only Internet Explorer supports the defer attribute.

<script type="text/javascript" defer="defer"></script>

Method 3: Dynamicly create DOM
JS origin is not required

These codes should be placed before the tag (near the bottom of the HTML file)

This loading method will prevent the onload event from being triggered before the loading is completed, and many of them are now The code of the page has to perform additional rendering work when onloading, so it will still block the initialization processing of some pages.

<script type="text/javascript">
    function downloadJSAtOnload() {
        var element = document.createElement("script");
        element.src = "defer.js";
        document.body.appendChild(element);
    }     
</script>

Method 4: Dynamicly create DOM during onload
Does not require js to come from the same source

These codes should be placed before the tag (near the bottom of the HTML file)

This loading method does not start asynchronous loading of js immediately, but starts asynchronous loading only at onload. This solves the problem of blocking the onload event from being triggered.

<script type="text/javascript">    
function downloadJSAtOnload() {        
var element = document.createElement("script");
        element.src = "defer.js";        
        document.body.appendChild(element);
    }    
    if (window.addEventListener)        
    window.addEventListener("load", downloadJSAtOnload, false);    
    else if (window.attachEvent)        
    window.attachEvent("onload", downloadJSAtOnload);    
    else window.onload = downloadJSAtOnload;
</script>

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