Code Smell 139 - Business Code in the User Interface

Validations should be on the interface, or not?

TL;DR: Always create correct objects in your back-ends. UIs are accidental.

Problems

• Security problems

• Code Duplication

• Testability

• Extensibility to APIs, micro-services, etc.

• Anemic and mutable objects

• Bijection Violation

Solutions

1. Move your validations to the back-end.

Context

Code Duplication is a warning for premature optimization.

Building a system with UI validations might evolve to an API or external component consumption.

We need to validate objects on the back-end and send good validation errors to client components.

Sample Code

Wrong

<script type="text/javascript">

  function checkForm(form)
  {
    if(form.username.value == "") {
      alert("Error: Username cannot be blank!");
      form.username.focus();
      return false;
    }
    re = /^\w+$/;
    if(!re.test(form.username.value)) {
      alert("Error: Username must contain only letters, numbers and underscores!");
      form.username.focus();
      return false;
    }