The async attribute on a <script> tag specifies that the script is executed asynchronously, that is, whenever it is available -- irrespective of the page load progress.
This only works for external scripts.
An async attribute on a <script> element.
<script src="/tutorial/script-async.js" async>
</script>
Here is the script-async.js script file. It displays an alert box when executed.
alert("This alert is executed asynchronously.");
The async attribute specifies that the script file is executed once it is available.
This attribute only works with external script files, and only when src attribute is present.
External script files can be executed in the following 3 ways:
<script src="URL" async>
Here is when async support started for each browser:
 Chrome
|
8.0 | Dec 2010 |
 Firefox
|
3.6 | Jan 2010 |
 IE/Edge
|
10.0 | Sep 2012 |
 Opera
|
15.0 | May 2013 |
 Safari
|
5.1 | Oct 2011 |
