What is the difference between async and defer in JavaScript?

What is the difference between async and defer in JavaScript?

Both async and defer are attributes for script tags in HTML 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 the order they appear in the document after the HTML is fully parsed.

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

Sure! Use async for scripts that are independent and don’t rely on other scripts, like analytics. Use defer for scripts that depend on the DOM being fully loaded, like your main application scripts.

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?

Yes, using async can improve performance by allowing scripts to load in parallel, while defer ensures scripts run in order without blocking HTML parsing.