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HTML <script> async Attribute

HTML script Tag Reference HTML <script> tag

Example

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

<script src="demo_async.js" async></script>

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Browser Support

Internet Explorer Firefox Opera Google Chrome Safari

The async attribute is supported in Internet Explorer 10, Firefox, Opera, Chrome, and Safari.

Note: The async attribute of the <script> tag is not supported in Internet Explorer 9 and earlier versions.


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 is present: 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 is present: 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.


Differences Between HTML and XHTML

In XHTML, attribute minimization is forbidden, and the async attribute must be defined as <script async="async">.


Syntax

<script async>


HTML script Tag Reference HTML <script> tag

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