url - Why can protocol be omitted from absolute paths on a webpage?

Question

I recently ran across a website that had some interesting styling on a select element. I went to investigate and found this (names changed to protect the innocent):

<script type="text/javascript" src="//www.domain.tld/file.js"></script>

It works despite HTTP: being omitted. What is the purpose of leaving off the protocol?

Answer 1

It will use the protocol you're already using. Useful for sites with both https and http versions.

So if you're on https://www.domain.tld/file.js the script will be https://www.domain.tld/file.js.

If you're on http://www.domain.tld/ the script will be http://www.domain.tld/file.js.