When it comes to including external JavaScript files in your web page, you might have come across the tag with the attributes
async
and defer
. These attributes can be used to control how the browser loads and executes the JavaScript code.
Understanding async and defer
The async
and defer
attributes are used to specify that the script should be executed asynchronously. This means that the script will not block the rendering of the page and will be downloaded and executed in the background while the rest of the page continues to load.
However, there is a subtle difference between the two:
async
: The script will be downloaded asynchronously and executed as soon as it is available. The order of execution may not be guaranteed if multiple scripts are included with theasync
attribute.defer
: The script will be downloaded asynchronously but will only be executed after the HTML document has been fully parsed. Multiple scripts with thedefer
attribute will be executed in the order they appear in the document.
When to use async and defer
The choice between async
and defer
depends on the specific requirements of your JavaScript code and how it interacts with the rest of the page. Here are some scenarios where each attribute can be useful:
Use async when:
- The script does not depend on any other scripts or the DOM.
- The script is self-contained and does not rely on other parts of the page.
- Order of execution is not important.
Use defer when:
- The script depends on other scripts or the DOM.
- The script needs to be executed in a specific order.
- The script needs to manipulate the DOM or interact with other page elements.
Code examples
Async example:
Defer example:
Final thoughts
Choosing between async
and defer
attributes for your tags can greatly impact the performance and behavior of your web page. It is important to understand the differences and choose the appropriate attribute based on your specific requirements.
Remember, if your script relies on other scripts or the DOM, it is generally safer to use the defer
attribute to ensure proper execution order.
By utilizing the async
and defer
attributes effectively, you can optimize the loading and execution of JavaScript code, resulting in a faster and more efficient web page.
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