Synchronous

<script src="path/to.js"></script>

Standard practice is to place JavaScript <script> tags just before the closing </body> tag. Loading your scripts last allows your site's visuals to show up more quickly and discourages your JavaScript from trying to interact with elements that haven't loaded yet.

Asynchronous

<script src="path/to.js" async></script>

Another alternative, when the Javascript code being loaded is not necessary for page initialization, it can be loaded asynchronously, speeding up the page load. Using async the browser will load the contents of the script in parallel and, once it is fully downloaded, will interrupt the HTML parsing in order to parse the Javascript file.

Deferred

<script src="path/to.js" defer></script>

Deferred scripts are like async scripts, with the exception that the parsing will only be performed once the HTML is fully parsed. Deferred scripts are guaranteed to be loaded in the order of declaration, same way as synchronous scripts.

<noscript>JavaScript disabled</noscript>

The <noscript> element defines content to be displayed if the user has scripts disabled or if the browser does not support using scripts. The <noscript> tag can be placed in either the <head> or the <body>.