One of the key factors that can significantly impact a website's load time is the way JavaScript is loaded. Two common methods for loading JavaScript are using the async and defer attributes. But what do these attributes do, and when should you use them? Let's dive in and find out.
What is Async?
The async attribute is used with a <script> tag to indicate that the script should be executed asynchronously.
This means that the script is downloaded in the background without blocking the rendering of the page. Once the script is downloaded, it executes immediately, potentially interrupting the parsing of the HTML.
This is ideal for scripts that are independent and do not rely on any other scripts or the DOM being fully loaded.
What is Defer?
The defer attribute, on the other hand, also allows the script to be downloaded in the background, similar to async.
However, scripts with the defer attribute won't execute until the HTML document has been fully parsed, just before the DOMContentLoaded event.
This ensures that scripts do not interrupt the rendering of the page and are executed in the order they appear in the HTML. This is particularly useful for scripts that rely on the DOM being fully loaded or depend on other scripts.
Async vs Defer: The Differences
While both async and defer allow scripts to be downloaded in the background without blocking rendering, their execution behavior differs. Scripts with the async attribute execute as soon as they're loaded, in no particular order.
This means that if two async scripts are loaded, the smaller one could potentially execute before the larger one, even if it appears later in the HTML.
On the other hand, defer'd scripts maintain their relative order. They are executed in sequence, just before the DOMContentLoaded event, regardless of their size or when they finish downloading.
This makes defer a better choice for scripts that depend on each other or on the DOM being fully loaded.
Which is Better?
The choice between async and defer depends on the specific needs of your script. If your script is independent and should run as early as possible, async is the way to go.
However, if your script needs to manipulate the DOM or depends on other scripts, defer is a safer choice. It ensures that your script won't execute until all the HTML is parsed and other scripts have run. Defer will also deliver better performance than Async.
Can You Use Both?
Technically, you can use both async and defer attributes on the same script tag. However, this is not recommended. Modern browsers that support both attributes will prioritize async, causing the defer attribute to be ignored.
It is not recommended to use both async and defer on the same script tag.
This could lead to unpredictable behavior, especially if your script relies on the DOM being fully loaded or other scripts having run. Therefore, it's best to use these attributes intentionally and purposefully, depending on your script's specific needs.
Conclusion
In conclusion, both async and defer offer ways to improve page load times by changing how JavaScript is loaded and executed.
By understanding the differences between them and their appropriate use cases, you can make informed decisions that enhance your website's performance and user experience.
Remember, the best choice depends on your specific needs and the nature of your scripts.