How to have JS Website interact with a mutable file

I learned that I can't have a file inside the bundle if I want it to be able to be changed while the app is running. If I have a file stored outside, how could I let an HTML website access that JS file? I'm using WKWebView to run a local website. The JS would need to be accessible by the website before it loads, because it is a configuration file that is needed on load.

How to have JS Website interact with a mutable file
 
 
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