The libraries I was thinking of sharing are all ones that are tightly coupled to the specific pages they would be on and all related to UI.
Things like form validation, user input normalization, navigation options. There are ton of small data manipulation functions that you either have to maintain in both javascript and your server side language and cause some weird bugs when one version doesn't behave exactly the same as the other or else you need to make a bunch of unnecessary ajax calls just to avoid reimplementing things in js.
There is much deeper integration you could do but I think we're on the same page as to why that's a bad idea.
Things like form validation, user input normalization, navigation options. There are ton of small data manipulation functions that you either have to maintain in both javascript and your server side language and cause some weird bugs when one version doesn't behave exactly the same as the other or else you need to make a bunch of unnecessary ajax calls just to avoid reimplementing things in js.
There is much deeper integration you could do but I think we're on the same page as to why that's a bad idea.