Write the form validation logic in the model and let formwell show the errors in the view as soon as the user enters something in the input fields
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Backbone Formwell

Simple Model-View Form Validation

Write validation logic in the Model and let Formwell show the errors in the View as soon as the user types something in the input fields


The goal is to have easy form validation for Backbone applications.

It doesn't have fancy methods to implement the validation logic, it doesn't have fancy declarative bindings and it doesn't have fancy view helpers to generate the form markup. It is just a simple convention to easily show model validation errors in the form.


Include the backbone.formwell script after backbone. For example:

<script type="text/javascript" src="backbone.min.js"></script>
<script type="text/javascript" src="backbone.formwell.min.js"></script>


Create an instance of Backbone.Formwell with a reference to the View and the Model:

MyView extends Backbone.View
  el: 'form'

  initialize: ->
    this.formwell = new Backbone.Formwell(model: this.model, view: this)

Link each input, textarea or select tag with the respective model attribute using the html name attribute in the template:

  <input type="text" name="firstName" value=""/>

Don't implement Backbone.Model validate method, and don't override isValid. Those methods need to be left untouched, because we want set and save to work properly even if the model is invalid, in order to keep the form and the model attributes in sync. (Backbone methods set and save depend on the validation method in a way that is too confusing for a dinamic form, the easiest workaround I could find is to just avoid using validate). Update: newer versions of Backbone don't block the set method with the validation, but I still prefer to have a different method for full compatibility.

Formwell will expect the model to have the method validateModel instead. Create the method validateModel in the model. If model.attributes are valid, don't return anything from validateModel (undefined, null, false or ''); if they are invalid, return an errors object, where keys are the invalid attribute names and the values the error messages.

For example:

User extends Backbone.Model

  validateModel: ->
    errors = {}
    errors['firstName'] = 'Can not be blank' if not this.get('firstName')
    return errors unless _.isEmpty(errors)

Whenever a form input changes, use formwell.set(attrName, value) instead of model.set(attrName, value)

On form submit, use formwell.validate() before saving the model to ensure there are no errors left in the form.


class Form extends Backbone.View
  el: 'form'

    "change input, textarea, select": "validateInputAndSet"
    "submit": "validateFormAndSave"

  initialize: ->
    @formwell = new Backbone.Formwell(model: @model, view: @)

  validateInputAndSet: (event) ->
    $input = $(event.currentTarget)
    @formwell.set($input) # use formwell.set instead of model.set

  validateFormAndSave: (event)->
    if errors = @formwell.validate() # show errors left
      alert("Can not save because there are errors in the form")

When the user changes a form input, formwell will set the value in the model attribute with the same name.

If there is an error in that attribute, it will show the error message in the form.


Errors, position and Styles

Formwell will create a DOM element to show the error right next to the invalid input.

For example, given this input:

<input type="text" name="firstName" value=""/>

If the user unfocus the input with an invalid value, formwell will create the error element:

<input class="with-error-message" type="text" name="firstName" value=""/>
<span class="error-message" data-name="firstName">Can not be blank</span>

If the error element already exists, it will not create a new one, so create it on your own template if you want to show the error message in a different place, just be sure to include the css class "error-message" and set the data-name attribute to the same referenced model attribute name.

When there is an error in an input field, formwell will also add the class "with-error-message" to the input, and also to the parent with class "form-row" if any.

For example, given this markup:

<div class="form-row">
  <label>First Name</label>
  <div class="input-wrapper">
    <input type="text" name="firstName" value=""/>
    <p class="hint">Please don't use a fake name</p>
  <div class="error-message" data-name="firstName" style="display:none"></div>

If the user unfocus the input with an invalid value, formwell will add the error message and error class:

<div class="form-row with-error-message">
  <label>First Name</label>
  <div class="input-wrapper">
    <input class="with-error-message" type="text" name="firstName" value=""/>
    <p class="hint">Please don't use a fake name</p>
  <div class="error-message" data-name="firstName">Can not be blank</div>

Use this to style the error messages.

Errors on the model itself

You don't need an input or model attribute in order to show an error. You can put something like this in your markup:

<span class="error-message" data-name="base" />

and then add error messages non related to a specific attribute in the key "base", for example the validate method could return something like {base: "There are errors on this form"} and it will be shown in the related element.

Nested models or lists

Formwell can only set first level attributes (using model.set). But you can add a "change" event listener in the model to assign the real model value (or override set, but is not recommended).

The attribute name will be used only to link the model with the form. For example, if you have a model User with a nested Address model with several attributes like city, state, etc. You could set the input name to address[city], and return something like {"address[city]": "city does not exist"} in the validation errors object.

But to actually set the value in the nested address model, you need to manually set the nested attribute value, or use the change event listener to do it automatically, for example:

User extends Backbone.Model
  initialize: ->
    this.on 'change', (model, options) ->
      for attrName, value of options.changes
        # Set nested values in address
        if m = attrName.match(/^address\[(.+)\]/)
          addressAttrName = m[1]
          model.get("address").set(addressAttrName, value)


You should use your own library for this, and implement the The validateModel method to return the error messages already translated.

Do not validate some fields

Backbone View uses jQuery (or Zepto) selectors in the events object, those selectors are pretty powerful. Just filter out the elements that you don't want to validate, for example:

MyView extends Backbone.View
    'change input': 'setInputValue'

  setInputValue: (event) ->
    $input = $(event.currentTarget)
    attrName = $input.attr('name')
    value = $input.val()
    if $input.hasClass 'skip-validation'
      @model.set(attrName, val)
      @formwell.set(attrName, val)


Github makes it easy! open a new issue or branch the project and send a pull request.


  • 1.0.0 (Oct 11, 2013): Find .form-field element from the error element, allowing to style the error without an actual input. Bump to v1.0.0 since this was working on production environments with no problems.
  • 0.0.3 (Sep 04, 2012): Bugfix hide errors after correcting the form field
  • 0.0.2 (Aug 28, 2012): Bugfix sometimes show 'error' message when model.validateModel was returning undefined. Review Readme
  • 0.0.1 (Aug 27, 2012): Initial release


Written by Mario Izquierdo