Using Knockout with existing HTML content

When creating an MVVM JavaScript application, Knockout is one of the most used libraries. One of its core principles is that all rendering is done client-side. One disadvantage is that this makes indexing the website harder. Another is that client-side rendering usually takes longer than server-side rendering.

The Knockout pre-rendered library adds two new binding handlers to Knockout that allow observables to be initialized from pre-rendered HTML content:

  • init: initialize an observable to a value retrieved from existing HTML content.
  • foreachInit: wraps the foreach binding, with the observable array’s elements bound to existing HTML elements.

Init binding

Suppose we have the following view model:

function ViewModel() {
  this.name = ko.observable();
}

Normally, you’d bind the name observable to an HTML element as follows:

<span data-bind="text: name">Michael Jordan</span>

Once the binding has been applied however, the text within the <span> element will be cleared, as the bound observable did not have a value (existing HTML content is ignored).

We can fix this by specifying the init binding handler before the text binding handler:

<span data-bind="init, text: name">Michael Jordan</span>

Now, the text within the <span> element is left unchanged. This is due to the init binding handler setting the observable’s value to the text content of the bound element. As Knockout binding handlers are executed left-to-right, when the text binding executes the init binding will already have initialized the observable.

You can combine the init handler with any binding, as long as you ensure that it is listed before the other bindings:

<span data-bind="init, textInput: name">Michael Jordan</span>

<!-- This binding will use the "value" attribute to initialize the observable -->
<input data-bind="init, value: name" value="Larry Bird" type="text" />

Converting

By default, the init binding will set the observable’s value to a string. If you want to convert to a different type, you can specify a convert function:

<span data-bind="init: { convert: parseInt }, text: height">198</span>

Now, the observable’s value will be set to what the convert function, with the innerText value as its parameter, returns.

Custom conversion

It is also possible to use your own, custom conversion function. You could, for example, define it in your view model:

function CustomConvertViewModel() {
  this.dateOfBirth = ko.observable();

  this.parseDate = function(innerText) {
    return new Date(Date.parse(innerText));
  }
}

You can then use this custom convert function as follows:

<span data-bind="init: { convert: parseDate }, text: dateOfBirth">

Virtual elements

You can also use the init binding handler as a virtual element:

<!-- ko init: { field: name } -->Michael Jordan<!-- /ko -->

Converting works the same as before:

<!-- ko init: { field: height, convert: parseInt } -->198<!-- /ko -->

Note that we now need to explicitly specify the field parameter, which points to the observable to initialize. In our previous examples, the init binding was able to infer this due to it being combined with the text, textInput or value binding and using the observable they were pointing to. As a consequence, the following bindings are equivalent:

<span data-bind="init, text: name">Michael Jordan</span>
<span data-bind="init: { field: name }, text: name">Michael Jordan</span>

Although you’d probably not need it, you could have the init handler initialize a different observable than the one text, textInput or value binding uses:

<span data-bind="init: { field: name }, text: year">Michael Jordan</span>

The result of this binding is that the name observable will have its value initialized to "Michael Jordan", after which the element will be bound to the year observable.

Foreach init binding

This binding handler wraps the foreach binding, but instead of creating new HTML elements, it binds to existing HTML elements. Consider the following view model:

function PersonViewModel() {
  this.name = ko.observable();
}

function ForeachViewModel() {
  this.persons = ko.observableArray();

  this.persons.push(new PersonViewModel());
  this.persons.push(new PersonViewModel());
  this.persons.push(new PersonViewModel());
}

We can bind the elements in the persons observable array to existing HTML elements by using the foreachInit binding handler:

<ul data-bind="foreachInit: persons">
  <li data-template data-bind="text: name"></li>
  <li data-init data-bind="init, text: name">Michael Jordan</li>
  <li data-init data-bind="init, text: name">Larry Bird</li>
  <li data-init data-bind="init, text: name">Magic Johnson</li>
</ul>

There are several things to note:

  1. There must be one child element with the data-template attribute. This element will be used as the template when new items are added to the array.
  2. Elements to be bound to array items must have the data-init attribute.
  3. You can use the init binding handler to initialize the array items themselves.

Template

You can also use a template that is defined elsewhere on the page:

<ul data-bind="foreachInit: { name: 'personTemplate', data: persons }">  
  <li data-init data-bind="init, text: name">Michael Jordan</li>
  <li data-init data-bind="init, text: name">Larry Bird</li>
  <li data-init data-bind="init, text: name">Magic Johnson</li>
</ul>

<script type="text/ko-template" id="personTemplate">
  <li data-template data-bind="text: name"></li>
</script>

Create missing array elements

Up until now, we assumed that the observable array already contained an item for each HTML element bound inside the foreachInit section. However, you can also have the foreachInit binding handler dynamically add an element for each bound element.

Take the following view model:

function ForeachDynamicViewModel() {
  this.persons = ko.observableArray();

  this.addPerson = function() {
    this.persons.push(new PersonViewModel());
  }
}

Note that the persons observable array does not contain any elements, but that it does have a function to add a new element to the array.

We can use the foreachInit binding handler as follows:

<ul data-bind="foreachInit: { name: 'personTemplate', createElement: addPerson }">  
  <li data-template data-bind="text: name"></li>
  <li data-init data-bind="init, text: name">Michael Jordan</li>
  <li data-init data-bind="init, text: name">Larry Bird</li>
  <li data-init data-bind="init, text: name">Magic Johnson</li>
</ul>

What happens is that for each element with the data-init attribute, the function specified in the createElement parameter is called.

Installation

The best way to install this library is using Bower:

bower install knockout-pre-rendered

You can also install the library using NPM:

npm install knockout-pre-rendered --save-dev

The library is also available from a CDN.

Demos

Here are some JSBin demo’s that show how to use the binding handlers:

Acknowledgements

Many thanks go out to Brian M Hunt, which fastForEach binding handler formed the basis of the foreachInit binding handler.

Conclusion

For newly-developed Knockout applications, you probably don’t need this library, unless you worry greatly about search engine indexation. However, if you have an existing application that already serves HTML, this library will allow you to integrate Knockout in an easy and non-disruptive way.