What are the differences between defer and async attributes in script tags?

What are the differences between defer and async attributes in script tags?

The defer attribute tells the browser to download the script while the HTML is being parsed, but it will execute the script only after the document has been fully parsed. In contrast, the async attribute allows the script to be downloaded and executed as soon as it's available, which can happen before the HTML is fully parsed.

When should I use defer instead of async?

You should use defer when the script relies on the DOM being fully loaded, such as when it manipulates elements on the page. Use async for scripts that are independent and do not rely on the DOM, like analytics or ads.

Can I use both attributes together?

No, you cannot use both defer and async on the same script tag. They are mutually exclusive, and the browser will ignore one if both are present.

What happens if I don't use either attribute?

If you don't use either attribute, the script will block the parsing of the HTML until it is downloaded and executed, which can lead to slower page load times.