How does the async attribute work inside script elements and how does it differ from the defer attribute?
The async and defer attributes in HTML script elements play a crucial role in how JavaScript files are loaded and executed in web pages. Understanding them can improve your website’s performance and user experience. When you include a script in your HTML file, it looks like this:
<script src="example.js"></script>When the browser finds this script tag, it stops parsing the HTML, downloads the script, executes it, and then continues parsing the HTML. This can slow down the loading of your web page, especially if you have large scripts.
This is where the async and defer attributes come in. They provide ways to load scripts more efficiently.
Let’s start with the async attribute:
<script src="example.js" async></script>By adding the async attribute to a script tag, the browser will continue parsing the HTML while the script is being downloaded. Once the script is fully downloaded, the browser will pause HTML parsing, execute the script, and then resume parsing the HTML. This can significantly speed up page loading.
It’s important to note that async scripts are executed as soon as they’re downloaded, which means they might not run in the correct order which we desire. This is where the defer attribute comes in for the rescue. Let’s look at how the defer attribute looks like:
<script src="example.js" defer></script>The defer attribute is similar to async attribute. However, defer scripts are not executed immediately after they’re downloaded. Instead, they wait until the HTML parsing is complete. Furthermore, defer scripts execute in the order they appear in the HTML code.
In short, use async for scripts where the order of execution doesn’t matter, and use defer when you need to ensure that scripts run in the correct order. Both attributes can significantly improve page load times by allowing the browser to continue parsing HTML while scripts are being downloaded in the background.