All You Need to Know About Async & Defer
Defer are attributes that are commonly used within HTML tags, especially the
script tag. We typically use the
script tag is blocking in nature, which means that when a browser encounters it, it automatically delays rendering DOM elements until the script finishes loading.
Therefore, if a script is heavy, the entire page will remain blank until the script has loaded, which can result in a poor user experience. Thanks to
Defer, we have a way to get out of this situation.
In this article, we will elaborate, why and when to use
Defer to get the best results out of it.
<div>Before Script Tag </div> <script defer src="app.js" ></script> <!-- this div will be visible after app.js execution--> <div>After Script Tag</div>
Before moving into the
Defer, hold a for a sec, please. Because some of might wonder what about using a script at the end of the HTML document? Well, my friend, this might a solution but not a sustainable solution.
When an HTML page is rendering slowly at that moment some functionality or code may break because the script isn't immediately available yet. So we need to load script parallely when dom content is also rendering.
To simplify things, we previously explained that the script tag blocks the rest of the DOM until it finishes executing. However, there's a catch: while scripts can be loaded or retrieved without blocking the rest of the HTML, their execution still blocks it.
To get around this, we can use the
Defer attribute. By using
Defer, our script will load asynchronously alongside DOM parsing, and then execute before the
<div>Before Script Tag </div> <script defer src="app.min.js" ></script> <!-- this div will be visible immediately --> <div>After Script Tag</div>
Suppose, you have multiple scripts in your HTML document. Out of them, one script may be crucial for the functionality of the webpage, so it needs to be executed first before any other script. In this case, we must use
Defer as it maintains the relative order of the scripts, even if one script requires more time than the others.
<div>Before Script Tag </div> <script defer src="app.max.js" ></script> <script defer src="app.min.js" ></script> <!-- this div will be visible immediately --> <div>After Script Tag</div>
Here, if Script
app.min.js finishes its loading before
app.max.js, it will still have to wait for
app.min.jsto be loaded and finished before executing.
Another important fact is that the
Defer attribute only applies to external scripts with a
src attribute, indicating that it is not applicable to inline scripts.
Async Scripts are completely different from
Async Scripts are fully independent, meaning they don't wait for the
DomContentLoaded event and the
DomContentLoaded event doesn't wait for Async Scripts.
Additionally, Async Scripts don't maintain the relative order of scripts. They work on a first-come, first-served basis, meaning that scripts with the shortest loading/execution time will get priority, regardless of their position.
<div>Before Script Tag </div> <script async src="app.max.js" ></script> <script async src="app.min.js" ></script> <!-- this div will be visible immediately --> <div>After Script Tag</div>
Async can be tricky, and it requires careful consideration. Since
Async Scripts don't wait for the
DOMContentLoaded event, there is a possibility that the HTML DOM may be loaded faster than
Async Scripts execution, potentially causing functionality issues.
Async doesn't maintain the relative order of scripts, less important scripts may be loaded before critical ones. Therefore, it is recommended to use
Async Scripts for independent features like chat or comment sections, counters, ads and so on, as they don’t depend on our scripts, and our scripts shouldn’t wait for them, rather than critical scripts.
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