Yes, there is at least one tool that does this. There is a Ruby on Rails plugin called Client-Side Validation. It generates client-side Javascript validation ba

Yes, there is at least one tool that does this. There is a Ruby on Rails plugin called Client-Side Validation. It generates client-side Javascript validation based on the existing validation in the model, which the server already checks against. You can find it here: http://agilewebdevelopment.com/plugins/client_side_validation http://agilewebdevelopment.com/plugins/client_side_validatio.... I haven't tried it myself, so I can't vouch for how well it works. There also may be other, newer libraries for doing this out there. I certainly agree that elimination of this repetition would be very helpful when developing websites. If the code can be generated automatically for you, there are very few situations where you wouldn't want client-side as well as server-side validation. (Some of those few situations might include expecting the user to have a computer too slow for Javascript or being unable to expose the validation method to the user for security reasons.)