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Difference between async and defer, Use of async and defer

As you want to know what is the difference between async and defer experts and when and where they should be used.

As you want to know what is the difference between async and defer experts and when and where they should be used. So for this you read this post, maybe you can get some information about async and defer by this article.

Why are async and defer used?

As you should know that async and defer attributes are used between to adjust the loading of a script or JavaScript. However, both features cannot be used simultaneously in a script.
When a browser loads a webpage on a user’s device, it reads the coding of that webpage head to line at a very rapid rate, which is called rendering, and thus renders that webpage. Is. In this process HTML, CSS and Script or JavaScript of that webpage is read.

Use of async and defer

Since when a webpage contains a lot of JavaScript or scripts, the browser takes a lot of time to display the page, because the functions of these scripts have to be read and processed before displaying them. While reading a script, the rendering process of the browser is stopped or blocked until the browser can process that script. When this happens, the speed of that webpage slows down.

So to avoid this, so that a script does not block the rendering process of the browser, scripts are used with the async or defer attributes, so that that script is rendered or processed later in the rendering process.

The async and defer attributes are optional attributes for the <script> tag that instruct the browser to download the indicated file at a lower priority in the background without interrupting the page rendering process.

This makes the async and defer attributes a simple way to eliminate the render blocking effect of JavaScript resources, as well as placing <script> references in the section to prompt them to download as early in the loading process as possible.

Any resource provided on a page is first downloaded and then displayed or played on the page. Whereas async and defer have the same loading behavior – loading in the background without render blocking.

As well as eliminating the render blocking effect of JavaScript, another advantage of the async and defer features is that they are loaded with a lower priority than traditional <script> resources which may help conserve bandwidth for other resources. Which are more important for the initial display of the webpage.

Where not to use async and defer attributes

If you apply the async or defer attribute to those JavaScript resources, it can often break some of the resources that depend on the resources first appearing in the document or the primary infrastructure operating resources of that webpage.

The async allows the HTML parser (such as a visitor’s browser) to download JavaScript on one hand while parsing or rendering the rest of the HTML. So it doesn’t stop parsing completely when the web page is downloaded. However, this will prevent the HTML parser to execute the script after it has been downloaded.

Function of defer

The defer – waits for the script to be executed until the HTML parser has finished parsing the HTML in order to download the JavaScript to the HTML parser while parsing the rest of the HTML for that webpage. The advantage of using defer is that your script is guaranteed to be executed in the order it appears.

The async and defer both load JavaScript asynchronously without causing a render block, but async executes as soon as possible, while defer runs in sequence at the end of the loading process, just before the DOM content.

The effect of the defer attribute is similar to that of placing a traditional <script> reference at the bottom of HTML before the closing </body> tag. The advantage of defer is that <script> can be placed in the to be searched and can start loading sooner if the page has fewer resources.

When to use async

Using async is best for very high priority JavaScript resources that should be loaded as early as possible in the loading process.
In the interest of page speed, async should be used with caution and generally only for files on which the initial rendering of the page depends.

When to use defer

The defer is ideal for generic JavaScript resources that are not critical to the initial rendering of the page and may be run later in the loading process.
The use of defer attribute is the best option for optimum page speed and user experience in most of the cases. Especially for slow mobile devices, Defer allows the browser to display content to the user sooner by rendering the most important HTML and CSS first and delaying JavaScript execution to a more appropriate stage in the loading timeline.
How to add async or defer attribute to script
The async or defer attribute in a script also depends on the type of CMS, but the async and defer attributes are often added to the script, something like this:

<script async src="super-high-priority.js"></script>
<script defer src="general-stuff.js"></script>
<script async='async' src='' type='text/javascript'/>
<script async='async' src=''/>
<script async='async' src=''/>
<script defer='defer' src=''/>

Whichever method your theme supports, you can use the async or defer attribute.


The use of the async and defer attributes are good options for processing JavaScript that is blocking the render.

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