No it is a bad idea,
This is quite a subjective question, but personally, I wouldn't want a back-end framework to do this for me. It's better to keep a clean seperation between business logic, presentation, and client-side UI behaviours for a number of reasons:
Best practice of the day:
Keep frontend-code and backend code untangled as much as you can
Check out my PHP framework, PHP On Pie (http://phponpie.com) for an example of how to implement this properly. It keeps the JS and PHP separate, except when exporting JSON as shown above. However it also provides conventions for easy interoperation between the client and server via AJAX.
I believe you should keep languages apart. Even though they can complement each other. That way you can pick the implementation of said language and create a mix that fits you perfect.
2) You need server side side validation too, but you knew that, right?
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