With the addition of ES modules, there's now no fewer than 24 ways to load your JS code: (inline|not inline) x (defer|no defer) x (async|no async) x (type=text/javascript | type=module | nomodule) -- and each of them is subtly different.
This document is a comparison of various ways the <script> tags in HTML are processed depending on the attributes set.
If you ever wondered when to use inline <script async type="module"> and when <script nomodule defer src="...">, you're in the good place!
Note that this article is about <script>s inserted in the HTML; the behavior of <script>s inserted at runtime is slightly different - see Deep dive into the murky waters of script loading by Jake Archibald (2013)
