Something I’ve been learning (and teaching) in Software Engineering is that code duplication is the root of all evil. On the other hand, I find it quite hard to explain how this concept should be applied to the development of web apps.
Allow me to clarify… Input & data validation can be an important part of a web app. Sometimes this validation can be quite complex. For example, I worked on a puzzle editor and the validation consisted of checking whether an operation or a move was valid. Non-trivial rules then had to be checked.
Naturally, validation must be done server-side in order to ensure the consistency and quality of the stored data. However, it’s a must to do validation client-side to ensure a smooth user experience.
In most instances, client-side and server-side code are written in different languages (i.e. javascript/Python), so validation code has to be written twice. However, in my only experience with GWT/Java (Java on both sides), I found that a large portion of the validation code could be reused. This seemed to make everything easier: maintenance, refactoring, debugging…
So my question to you is: how do you manage issues related to code duplication in projects where the client-side and server-side languages are different?
Typically, it’s really hard to avoid duplicating the generated code, but a common approach is to use a code generator to build either the server or client side code so you only code one half of it. The most popular approach is writing the server-side common and then having the code generator build the JavaScript code for you. For instance, the language we use at my company is Coldfusion and Form-o-matic solves that problem for us. People have also approach the problem from the opposite direction by writing JavaScript which can be executed server side. I’d look for a framework that will do this for you.