Is it a good idea to implement business logic on the client side with JavaScript?
What kind of logic should be there? Validation Logic? Related to GUI?
What would you do if the same logic want to be used in another application (exposed) implementing it in JavaScript would mean you can't reuse that logic.
On the other hand having all logic on the server side would mean more requests to the server.
What do you think?
You can create reusable Javascript modules so there's no intrinsic barrier to resuing logic in several different rich uis. However, as has already been pointed out, you probably end up with duplication between the JavaScript and whatever you're using on the server (Java, PHP ...) - in the case of validation that's a trade-off between giving a performant user experience and complexity due to duplication.
I can imagine scenarios where you would choose to duplicate more than just validation. Consider computing a total order value: do we really want to make a server-side round trip for that? Sorting a list - we tend to do that happily in JavaScript, but we sorting can get interesting, specialised comparator functoions? Drawing the boundary may be quite tricky, computing discounts and sales tax?
In the end it's a judgement call, followed by careful understanding of consequences. If you duplicate logic then can you devise a test strategy that ensures consistency? With low volume systems you may be inclined to favour more server interactions and less duplication, but you may well make different decisions for a larger or more demanding user-base.