Asynchronous Script
Description
The default browser behavior when it encounters a script element
is to stop processing the page while it loads and executes the script.
Each script element is executed synchronously
in the order in which they are defined, nothing else happens when
the script is loading and running.
You can improve performance by using the async attribute.
For example, a self-contained script, which does not interact with the
elements in the HTML document, can be executed asynchronously.
When you use the async attribute,
the browser loads and executes the script asynchronously while it
continues to parse the other elements in the HTML, including other script elements.
Example
The following code shows the async attribute applied to a script element.
<!DOCTYPE HTML>
<html>
<head>
<script async src="simple2.js"></script>
</head>
<body>
<p>This is a test.</p>
<a href="http://java2s.com">java2s.com</a>
</body>
</html>
Note
The scripts with async attribute might not be executed
in the order in which they are defined.
This makes the async feature unsuitable for scripts that depend on
functions or values defined by other scripts.