How to add javascript library that doesn't exist on bower?

I’d like to add some javascript to my theme.

-custom JS that loads on every page
-JS library that doesn’t exist on bower

What is the best way to do it?

I’d like to keep core sage files untouched

Seems to me that old answers are outdated because grunt is not longer used

Take a look at:

There is a pull request that adds this to the documentation that I could use help on:

Why doesn’t it work by adding a line in manifest.json and the script in scripts library?

What’s the purpose and limits of manifest.json?

Not sure what you mean. That sounds like exactly how it works.

Didn’t work for me on Windows. Will try again on fresh install