Thanks for the answer, but my question isn't about user authentication/validation but validation of input from the user into my application.
A simple example, I set up a survey app. Several of the questions contain user input in the form of dates or numbers such as, date of birth, number of years living in the united states, years employed, etc. To help weed out bogus answers I want the fields to be validated/filtered. Date fields should be dates, numeric fields should only contain digits. In a traditional client server app I can validate this on the client side before submission in javascript, but I would still need to validate it on the server side as the clients input cannot be trusted. There is no guarantee that the validation code was ever run, that the user didn't modify the code as/before it was executed or that the user didn't simply post bogus data via a script.
Much more importantly, if I'm taking user input and then pushing that data back into the page for other users to see how do I prevent someone from submitting malicious javascript that is then executed by the next users browser when they view the document? The pitch is that my application is 100% client side javascript so it would seem to preclude the vast majority of apps that I can even think of since the users input would need to be processed/validated server side before I would be willing to store it or make it available for others to consume.