HTML <script> async Attribute
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>
Try it Yourself »
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 notasync
): The script is downloaded in parallel to parsing the page, and executed after the page has finished parsing - If neither
async
ordefer
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