For example, I want to turn: https://google.com/A7239J into... https://example.com/google.com/A7239J I want to be able to do this within the browser, using Javascript. Thanks!
If you mean to fetch resources using that URL as the base URL, then you can use the <base> element:
<!-- every URL will be relative to this value -->
<base href="https://upload.wikimedia.org/wikipedia/commons/">
<img src="./4/47/PNG_transparency_demonstration_1.png" width=300>
<img src="./c/cf/Black_hole_-_Messier_87.jpg" width=300>