What are the differences between async and defer in JavaScript?

Can you explain the difference between async and defer in JavaScript?

Sure! Both async and defer are attributes for the <script> tag that control how scripts are loaded. The main difference is that async scripts are executed as soon as they are downloaded, while defer scripts are executed in order after the HTML is fully parsed.

Can you give me an example of when to use each?

Absolutely! Use async for scripts that don't depend on other scripts or the DOM, like analytics. Use defer for scripts that need to run after the document is fully loaded, like those that manipulate the DOM.

What happens if I use both async and defer?

If both are used, the async attribute will take precedence, and the script will be executed as soon as it's downloaded, ignoring the defer behavior.

Are there any performance implications of using one over the other?

Yes, using async can improve performance for independent scripts, while defer can help maintain the order of execution for dependent scripts, which can also enhance performance.