What is the difference between async and defer in JavaScript?

What is the difference between async and defer in JavaScript?

The async and defer attributes are used in script tags to control how JavaScript files are loaded. async loads the script asynchronously, meaning it will be executed as soon as it's downloaded, without blocking the HTML parsing. In contrast, defer ensures that the script is executed only after the HTML document has been fully parsed.

When should I use async over defer?

You should use async when the script is independent and doesn't rely on other scripts or the DOM being fully loaded. Use defer when the script depends on the DOM being fully parsed or when it relies on other scripts.

Can I use both async and defer together?

No, you cannot use both async and defer on the same script tag. If both are present, only async will be applied.

What happens if I don't use either?

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