載入JavaScript code的最佳時機

 To summarize:

async and defer both instruct the browser to download the script(s) in a separate thread, while the rest of the page (the DOM, etc.) is downloading, so the page loading is not blocked by the scripts.

If your scripts should be run immediately and they don't have any dependencies, then use async.

If your scripts need to wait for parsing and depend on other scripts and/or the DOM being in place, load them using defer and put their corresponding <script> elements in the order you want the browser to execute them.