Giulio Canti

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What if your domain model could validate the UI for free?

Written by Giulio Canti on 12 Aug 2014

The Idea

Does the world need another validation library? Probably not, but I think of tcomb more as a tool to build fast and safe domain models. Since I want the models be the single source of truth and it’s hard to keep my models and the UI validation rules synced, I think there is still something to explore. The models should already express those validation rules so I only need to make them explicits. I’ll show you how to achive this goal with a simple example, a sign up form.


You can found a demo here.

[UPDATE] Implementation

For an implementation of the ideas exposed in this post see the tcomb-form library on GitHub.

The Model

Let’s design the domain model for the sign up process

// domain.js

// A Reddit like sign up:
// - username: required
// - password: required
// - email: optional

// a username is a string with at least 1 char
var Username = subtype(Str, function (s) {
  return s.length >= 1;

// a password is a string with at least 6 chars
var Password = subtype(Str, function (s) {
  return s.length >= 6;

// an email is a string that contains '@' :)
var Email = subtype(Str, function (s) {
  return s.indexOf('@') !== -1;

// sign up info
var User = struct({
  username: Username,
  password: Password,
  email: maybe(Email) // optional, can be `null`

// send signup info to server
User.prototype.signup = function () {
  $.post('/signup', JSON.stringify(this));

The View

I’ll use Bootstrap for this

<!-- signup.html -->

<script src="tcomb.js"></script>
<script src="domain.js"></script>
<script src="signup.js"></script>

<form role="form" method="post">
  <div class="form-group">
    <input type="text" id="username" placeholder="Username" class="form-control"/>
  <div class="form-group">
    <input type="password" id="password" placeholder="Password" class="form-control"/>
  <div class="form-group">
    <input type="text" id="email" placeholder="Email (optional)" class="form-control"/>
  <button class="btn btn-primary btn-block">Sign up</button>

The View Controller

This is the tricky part: the controller must validate the input and show visual feedback to the user that something went wrong. It’s straightforward to write a general validating function exploiting the meta.props hash of structs.

// signup.js

$('form').on('submit', function (evt) {
  // getting an instance of User means validation succeded
  var user = validate(User);
  if (user) {
    alert('Signup info sent.');

// configurable validating function
// - visual feedback is more fine grained
// - assume inputs are named like the struct props
function validate(Struct) {

  var values = {};
  var props = Struct.meta.props;
  var isValid = true;

  for (var id in props) {
    if (props.hasOwnProperty(id)) {
      var $input = $('#' + id);
      var value = values[id] = $input.val().trim() || null;
      if (!props[id].is(value)) {
        isValid = false;
      } else {

  return isValid ? new Struct(values) : null;

If you decide later that a username must be a string with at least 4 chars and you change the model accordingly, you don’t have to touch the view controller code.

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