23 5 / 2012

Data Binding Using data-* Attributes

Custom data-* attributes in HTML5 are pretty rad. They’re especially handy for stashing small amounts of data and retaining minimal state on the DOM. Turns out, they can also be used for one-way data binding!

I’ve been using a nifty trick in recent projects that I thought would be worth sharing. The technique is to use a data attribute to store values (i.e. the data model) and :before/:after pseudo elements to render the values as generated content (i.e. the view). I call it "poor man’s data binding" because it’s not true data binding in the traditional sense, but the semantics are similar. Count it!

Here we go:

<style>
  input {
    vertical-align: middle;
    margin: 2em;
    font-size: 14px;
    height: 20px;
  }
  input::after {
    content: attr(data-value) '/' attr(max);
    position: relative;
    left: 135px;
    top: -20px;
  }
</style>

<input type="range" min="0" max="100" value="25">

<script>
  var input = document.querySelector('input');

  input.dataset.value = input.value; // Set an initial value.

  input.addEventListener('change', function(e) {
    this.dataset.value = this.value;
  });
</script>

TRY IT

Notice the 25/100 updates as you move the slider, but the <input> is the only markup on the page.

The magic line is the content: attr(data-value) '/' attr(max). It uses CSS attr() to pull out the data-value and max attributes; both set using markup on the <input>. As those values change, the generated content is automatically updated. Sick data binding bro.

Really the only benefit of this technique is that we’re not including extraneous markup. For comparison, here’s the same gig, but using an extra element:

<input type="range" min="0" max="100" value="25"><span></span>

<script>
  var input = document.querySelector('input');
  input.addEventListener('change', function(e) {
    document.querySelector('span').textContent = this.value + '/' + this.max;
  });
</script>

Less elegant, but it works.

Last but not least, here’s a more complex example that uses CSS transitions to change the height of a div container when clicked. As the height changes, requestAnimationFrame() updates the data-height of the div and the pseudo element picks that up.

TRY IT

I’m sure if HTML was conceived in the age of web apps, we’d have proper DOM/JS data binding by now. Fortunately, initiatives like MDV and Web Components are on their way. One day this stuff will be a reality and native to HTML!

Data binding is a technique for automatically synchronizing data between two sources. On the web, data binding typically manifests itself as updating DOM (UI) in response to events: XHRs, user input, or other business logic doing its thing. Take the canonical todo list for example. When I mark an item as done, the completed count increments. When it’s unchecked, the count decrements. That’s data binding!

If you want true two-way data binding, checkout one of the popular MVC frameworks like Angular, Knockout, or Ember.

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