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 |
Example
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.
| Attribute | |||||
|---|---|---|---|---|---|
| async | Yes | 10.0 | 3.6 | Yes | Yes |
