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HTML5 <script> async Attribute

HTML5 script Tag Reference HTML5 <script> tag

Example

A script that will be run asynchronously as soon as it is available:

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

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Browser Support

Internet Explorer Firefox Opera Google Chrome Safari

The async attribute is supported in all major browsers, except Internet Explorer.

Definition and Usage

The async attribute is a boolean attribute.

When present, it specifies that the script will be executed asynchronously as soon as it is available.

Note: The async attribute is only for external scripts (and should only be used if the src attribute is present).

Note: There are several ways an external script can be executed:

  • If async="async": The script is executed asynchronously with the rest of the page (the script will be executed while the page continues the parsing)
  • If async is not present and defer="defer": The script is executed when the page has finished parsing
  • If neither async or defer is present: The script is fetched and executed immediately, before the browser continues parsing the page

Differences Between HTML 4.01 and HTML5

The async attribute is new in HTML5.

Syntax

<script async="async">

Note: The async attribute is a boolean attribute, and can be set in the following ways:

  • <script async>
  • <script async="async">
  • <script async="">
HTML5 script Tag Reference HTML5 <script> tag
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