Knockout Components to Structure Your Apps Better

Knockout 3.2.0 has recently been released. One of the biggest improvements is the introduction of Components, which along with Custom elements, allows you to break your app into reusable widgets, sections or pages.

Components combine a viewmodel and a template in an encapsulated package. A simple example could be a navigation bar or a progress panel. You can use "component binding"to bind these components to regular div elements, but there is a second, more powerful way to use them in your app - by using custom elements.

For example, you can define a component like so (note in a real project, you would have the view and the view model in separate files) -

ko.components.register('like-widget', {
    viewModel: function(params) {
        // Data: value is either null, 'like', or 'dislike'
        this.chosenValue = params.value;
        
        // Behaviors
        this.like = function() { this.chosenValue('like'); }.bind(this);
        this.dislike = function() { this.chosenValue('dislike'); }.bind(this);
    },
    template:
      '<div class="like-or-dislike" data-bind="visible: !chosenValue()">\
          <button data-bind="click: like">Like it</button>\
          <button data-bind="click: dislike">Dislike it</button>\
       </div>\
       <div class="result" data-bind="visible:chosenValue">\
          You <strong data-bind="text: chosenValue"></strong> it\
       </div>'
});

And then use it as so in your view -

<ul data-bind="foreach:products">
   <li class="product">
      <strong data-bind="text: name"></strong>
      <like-widget params="value: userRating"></like-widget>
   </li>
</ul>

Along with the corresponding view-model with property "products". The advantage being that the like-widget is now reusable in multiple places, and all the logic related to this piece is separately encapsulated. Breaking your app into several such components can make it vastly easier to build, understand and maintain. 

This looks very similar to the AngularJS philosophy of extending html elements by using custom components as well as the custom elements section of the WebComponents standard which is currently evolving. Developers are definitely excited about this. 

There are also several other improvements in this release -

• ko.pureComputed - a new specialisation of ko.computed with better performance and memory management. This feature is applicable only for computed observables whose evaluator does not cause any side effects, and whose value depends solely on the values of other observables in the application. In other words, when there are no hidden values or state outside of the dependency tracking infrastructure that affect the outcome of the evaluator.
• A new "textInput" binding which binds a <input> or <textarea> with a viewmodel property for two-way updates. This is superior to the "value" binding because it handles all kinds of user inputs such as autocomplete, drag-and-drop and clipboard events.
• "value" binding acts like "checkedValue" for checkboxes and radiobuttons. It has been a common mistake to use "value" instead of "checkedValue", so this change creates a fall-back option. "checkedValue" is still supported.

There are also several fixes - you can glance at the release notes for a full list of changes.

Knockout is a JavaScript MVVM library which comes with declarative bindings, 2-way updates between the DOM and the view-model, templating and a dependency tracking infrastructure.