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HTML async Attribute

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

Applies to

The async attribute can be used on the following element:

Elements Attribute
<script> async


Script Example

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

<script src="demo_async.js" async></script>
Try it Yourself »

Browser Support

The numbers in the table specify the first browser version that fully supports the attribute.

async Yes 10.0 3.6 Yes Yes