Optimizing JavaScript Inclusion: Async vs. Defer

JavaScript's Async vs. Defer: Which to Use and How to Add JS Files to HTML

Async:

Tells the browser to load and execute the JS file as soon as it's available, regardless of DOM parsing. This means the JS file could be executed before the DOM is fully loaded, which can lead to errors.

Defer:

Tells the browser to load the JS file in parallel with the HTML document, but to wait until the HTML document has finished parsing before executing it. This ensures the JS file will have access to all of the DOM elements it needs.

When to Use Async vs. Defer:

• Use async if the JS file is independent of the DOM. For example, if the JS file is used to load a third-party library or track analytics, it's safe to use async.
• Use defer if the JS file relies on the DOM. For example, if the JS file is used to manipulate DOM elements or add event listeners, it's best to use defer to ensure the JS file has access to all the DOM elements it needs.
• Avoid using async for critical JS files. If the JS file is critical to the loading and functionality of the page, it's best to avoid using async to ensure the JS file is executed before the page is rendered.

How to Add JS Files to HTML:

<script src="my-script.js" async></script>

Async will load and execute the JS file as soon as it's available.

<script src="my-script.js" defer></script>

Defer will load the JS file in parallel with the HTML document, but wait until the HTML document has finished parsing before executing it.

My Opinion:

Async and defer are powerful attributes that can improve the performance of web applications. However, it's important to understand the difference between the two before using them. In general, it's recommended to use defer unless you have a specific reason to use async.