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HTML <script> async Attribute

❮ HTML <script> tag

Example

A script that will be downloaded in parallel to parsing the page, and executed as soon as it is available:

<script src="demo_async.js" async></script>
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Definition and Usage

The async attribute is a boolean attribute.

If the async attribute is set, the script is downloaded in parallel to parsing the page, and executed as soon as it is available. The parsing of the page is interrupted once the script is downloaded completely, and then the script is executed, before the parsing of the rest of the page continues.

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 downloaded in parallel to parsing the page, and executed as soon as it is available (before parsing completes)
  • If defer is present (and not async): The script is downloaded in parallel to parsing the page, and executed after the page has finished parsing
  • If neither async or defer is present: The script is downloaded and executed immediately, blocking parsing until the script is completed

Browser Support

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

Attribute
async 8.0 10.0 3.6 5.1 15.0

Syntax

<script async>

❮ HTML <script> tag