The async
and defer
attributes in HTML <script> tags are used to control when and how scripts are executed in a web page, particularly in relation to the HTML parsing process.
-
Async attribute:
When the
async
attribute is present in a <script> tag, it tells the browser to download the script file asynchronously while continuing to parse the HTML document. Once the script is downloaded, it's executed without pausing the HTML parsing. This means that the script can be executed out of order concerning its position in the HTML document, leading to potential race conditions if scripts are dependent on each other's execution order.Example:
<script src="script.js" async></script>
-
Defer attribute:
The
defer
attribute, on the other hand, also allows the browser to download the script asynchronously. However, it defers the execution of the script until the HTML parsing is complete. Multiple scripts withdefer
will execute in the order they appear in the document, just before theDOMContentLoaded
event fires.Example:
<script src="script.js" defer></script>
-
Performance:
Both
async
anddefer
attributes can improve page load performance by allowing the browser to download scripts without blocking the parsing of the HTML content. This can speed up the initial rendering of the webpage, especially for scripts that are not essential for the initial display. -
Script Execution Order:
Understanding the difference between
async and defer
is crucial.defer
ensures scripts execute in order just before theDOMContentLoaded
event, whereasasync
doesn't guarantee the order of execution, potentially causing issues if scripts are interdependent. -
Dependency Management:
If scripts depend on each other or need to be executed in a specific order, using
defer
or careful placement of scripts in the document becomes important to maintain the required execution sequence.
Choosing between async and defer
depends on the script's nature and its relationship with other scripts and the DOM. It's essential to consider the impact on script execution order and potential dependencies while optimizing page load performance.