JS is often used for front end web development, but it can be utilized across the full stack of web architecture as well. This includes back-end services like web analytics, event tracking and even ad serving.
Async will download the script without blocking the page from being rendered while it is being fetched. However, it won’t be executed until all the other scripts on the page have been parsed and DOM content is in place. Therefore, it’s best to use async when your scripts don’t have any dependencies on other scripts.