Except in most cases validation cannot be complete on the client side, anything related to server side logic has to be additionally checked.

Validation is also two different concerns if you compare serverside vs clientside.

http://stackoverflow.com/questions/162159/javascript-client-...

Basically: Client side validation is about user experience, Server side is about security. I'm hard pressed to believe you can get both with the same code.

I also don't see how localStorage is relevant to code sharing between client and server.