We use HTML selection lists for rendering the enumeration attributes originalLanguage and otherAvailableLanguages in the HTML forms in createBook.html and upateBook.html. Since the attribute otherAvailableLanguages is multi-valued, we need a multiple selection list for it, as shown in the following HTML code:

<form id="Book">
 <div><label>ISBN: 
   <input name="isbn" type="text" /></label></div>
 <div><label>Title: 
   <input name="title" type="text" /></label></div>
 <div><label>Original language:
   <select name="originalLanguage"></select>
 </label></div>
 <div class="multi-sel"><label>Also available in:
   <select name="otherAvailableLanguages" 
    multiple="multiple" rows="4"></select>
 </label></div>
  ...
 <div><button type="submit" name="commit">Save
   </button></div>
</form>

While we define the select container elements for these selection lists in the HTML code of createBook.html and upateBook.html, we fill in their option child elements dynamically in the setupUserInterface methods in v/createBook.js and v/updateBook.js with the help of the utility method util.fillSelectWithOptions.

In the case of a single select element, the user's single-valued selection can be retrieved from the value attribute of the select element, while in the case of a multiple select element, the user's multi-valued selection can be retrieved from the selectedOptions attribute of the select element.

Notice that the div element containing the multiple selection list for otherAvailableLanguages has the class value "multi-sel", which is used for defining specific CSS rules that adjust the element's size.

Since the enumeration attributes category and publicationForms do not have more than seven possible values, we can use a radio button group and a checkbox group for rendering them in an HTML-form-based UI. These choice widgets are formed with the help of the container element fieldset and its child element legend as shown in the following HTML fragment:

<form id="Book">
 ...
 <fieldset data-bind="category">
   <legend>Category</legend></fieldset>
 <fieldset data-bind="publicationForms">
   <legend>Publication forms</legend></fieldset>
 <div><button type="submit" name="commit">Save
   </button></div>
</form>

Notice that we use a custom attribute data-bind for indicating to which attribute of the underlying model class the choice widget is bound.

In the same way as the option child elements of a selection list, also the labeled input child elements of a choice widget are created dynamically with the help of the utility method util.createChoiceWidget in the setupUserInterface methods in v/createBook.js and v/updateBook.js.

setupUserInterface: function () {
  var formEl = document.forms['Book'],
      origLangSelEl = formEl.originalLanguage,
      otherAvailLangSelEl = formEl.otherAvailableLanguages,
      categoryFieldsetEl = formEl.querySelector(
          "fieldset[data-bind='category']"),
      pubFormsFieldsetEl = formEl.querySelector(
          "fieldset[data-bind='publicationForms']"),
      saveButton = formEl.commit;
  // set up the originalLanguage selection list
  util.fillSelectWithOptions( origLangSelEl, LanguageEL.labels);
  // set up the otherAvailableLanguages selection list
  util.fillSelectWithOptions( otherAvailLangSelEl, 
      LanguageEL.labels);
  // set up the category radio button group
  util.createChoiceWidget( categoryFieldsetEl, "category", [], 
      "radio", BookCategoryEL.labels);
  // set up the publicationForms checkbox group
  util.createChoiceWidget( pubFormsFieldsetEl, "publicationForms",
      [], "checkbox", PublicationFormEL.labels);
  ...
},

Notice that like a selection list implemented with the HTML select element that provides the user's selection in the value or selectedOptions attribute, our choice widgets also need a DOM attribute that holds the user's single- or multi-valued choice. We dynamically add a custom attribute data-value to the choice widget's fieldset element for this purpose in util.createChoiceWidget.

Since choice widgets do not allow arbitrary user input, we do not have to check constraints such as range constraints or pattern constraints on user input, but only mandatory value constraints. This allows simplifying responsive validation in the UI.

In our example app, the enumeration attributes originalLanguage, category and publicationForms are mandatory, while otherAvailableLanguages is optional.

In the case of a mandatory single-valued enumeration attribute like originalLanguage rendered as a single selection list, we can enforce a choice, and thus the mandatory value constraint, by not offering an empty or void option among the option sub-elements of the select element. If the attribute is rendered as a radio button group, we can enforce a choice, and thus the mandatory value constraint, in the create use case by initially setting the checked attribute of the first radio button to true and not allowing the user to directly uncheck a button. In this way, if the user doesn't check any button, the first one is the default choice.

In the case of an optional single-valued enumeration attribute rendered as a single-selection list, we need to include an empty or void option (e.g., in the form of a string like "---"). If the attribute is rendered as a radio button group, we do not check any button initially and we need to allow the user to directly uncheck a button with a mouse click in a click event listener.

In the case of a mandatory multi-valued enumeration attribute like publicationForms rendered as a multiple-selection list or checkbox group, we need to check if the user has chosen at least one option. Whenever the user selects or unselects an option in a select element, a change event is raised by the browser, so we can implement the responsive mandatory value constraint validation as an event listener for change events on the select element, by testing if the list of selectedOptions is empty. If the attribute is rendered as a checkbox group, we need an event listener for click events added on the fieldset element and testing if the widget's value set is non-empty, as shown in the following example code fragment:

setupUserInterface: function () {
  ...
  // check mandatory value constraint for checkbox group
  pubFormsFieldsetEl.addEventListener("click", function () {
    var val = pubFormsFieldsetEl.getAttribute("data-value");
    formEl.publicationForms[0].setCustomValidity( 
       (!val || Array.isArray(val) && val.length === 0) ? 
            "At least one publication form must be selected!":"" );
  });
  ...
}

Notice that the HTML5 constraint validation API does not allow to indicate a constraint violation on a fieldset element (as the container element of a choice widget). As a workaround, we use the first checkbox element of the publicationForms choice widget, which can be accessed with formEl.publicationForms[0], for invoking the setCustomValidity method that indicates a constraint violation if its argument is a non-empty (message) string.