I have some experience with sharing js libraries between server side and client side. It can be nice but comes at a price:

you'll bring all the problems of client side javascript to the serverside without any of the nice client side js-frameworks that makes cross-browser coding more sane.

We're using rhino serverside with the latest CSV version giving us full ECMAScript 5 compatibility on the serverside - but for the parts that are also used clientside we can forget about all the nice features of Rhino. For our clientside code we're using jQuery, but again, when sharing with the server we can forget about all of jQuery's utility methods and so on...

It's worth doing but it is not at all as smooth as it could be...