Where do you record validation rules for form data in a web application?

Question

Say you have a web form with some fields that you want to validate to be only some subset of alphanumeric, a minimum or maximum length etc.

You can validate in the c

Answer 1

To keep validation rules in one place I use only server-side validation. To make it more user-friendly I just make an asynchronous post request to the server, and server returns error informations in JSON format, like:

{ "fieldName1" : "error description", 
"fieldName2" : "another error description" };

Form is being submitted if the server returned an empty object, otherwise I can use information from the server to display errors. It works much like these sign-up forms that check if your login is taken before you even submit the form, with two key differences: request is being sent onsubmit, and sends all field values (except input type="file").

If JavaScript validation didn't work for any reason, regular server-side validation scenario (page reload with error informations) takes place, using the same server-side script.

This solution isn't as responsive as pure client-side validation (needs time to send/receive data between client and server), but is quite simple, and you don't need to "translate" validation rules to JavaScript.