Every case of using custom JS for fields is pretty specific to what needs to be done and it’s really complicated to do anything generic. Every type of field needs different handling and that handling can be completely different when a field is in a repeater vs not in a repeater. Someone could make a full time job out of writing tutorials for it. I have some examples of getting and updating values with JS here https://github.com/Hube2/acf-dynamic-ajax-select-example might give you an idea of what’s involved.
