The use of asynchronous scripts for page speed are kind of a recent thing. In short, asynchronous script loading makes it possible to load scripts separately from your usual page rendering path. Using this type of script loading works wonders for your initial view loading times.
By using asynchronous scripts you ensure that loading these scripts cannot hinder the rendering of your web page in any way because they are loaded separately from the actual page. So even when loading a very slow script your web page can finish rendering without having to wait for the script to finish loading, the script is not considered part of the files the page needs to render the first view of your page. This means visitors can see your content no matter if the script has loaded or not.
You can’t load all scripts asynchronously. The reason for this is that some of the scripts are needed for the initial view of your web pages. These scripts have to load synchronously because your web page needs them to correctly display your web pages. So you can only load those scripts asynchronously which you don’t need for the initial view.
Using the Async tag (recommended)
<script src="yourscript.js" async></script>
** Async works on almost all browser versions but is not supported by some older browsers. Though this is not an issue anymore since most browsers make use of preload scanners today (even the older browsers) and can handle the scripts.
var resource = document.createElement('script');
resource.src = "//domain.com/thescript.js";
var script = document.getElementsByTagName('script');
Google provides us with great asynchronous script for many of their services. If you have implemented one of their scripts recently it’s almost certain you are already using their asynchronous scripts as this is their default now. But if you have older versions of these scripts on your site it’s likely you are using their old synchronous versions. There is no reason to use the synchronous scripts anymore, you should update them with the newer asynchronous versions. Here is more information how to do this for every particular Google service:
Most of the popular web services use asynchronous scripts for their external web services nowadays. Disqus, Pinterest, Facebook, Twitter and most other widely used services. So with most external services you use on your website you don’t really have to worry about loading them asynchronously, it’s already done for you unless you use their older codes.
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