Custom Elements

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Why Custom Elements?

Custom Elements allow authors to define their own custom HTML elements. Authors associate JavaScript code with custom tag names, and then use those custom tag names as they would any standard tag.

For example, after registering a special kind of button called super-button, use the super button just like this:


Custom elements are still elements. We can create, use, manipulate, and compose them just as easily as any standard <div> or <span> today.

Basic usage

As with any element, custom elements can be created in JavaScript or declared. Custom element names must always contain a dash (-).

Element registration

Before you can use a custom element, it needs to be registered. Otherwise, the browser considers it an HTMLElement.


To register a new custom element in JavaScript, invoke document.registerElement() somewhere in the page. As before, custom elements built this way work just like standard elements.

Here's the imperative version of the previous example:

var XFooPrototype = Object.create(HTMLElement.prototype);
XFooPrototype.createdCallback = function() {
  this.textContent = "I'm an x-foo!";
}; = function() {
  console.log('foo() called');

var XFoo = document.registerElement('x-foo', {
  prototype: XFooPrototype

Note: the prototype must be chained to HTMLElement.prototype (i.e. instanceof HTMLElement.prototype).

Extending existing elements

If you want to inherit from a specialized form of HTMLElement (e.g. HTMLButtonElement), declare the type using the extends option when calling document.registerElement():

Example extending button:

var XFooButtonPrototype = Object.create(HTMLButtonElement.prototype);
XFooButtonPrototype.createdCallback = function() {
  this.textContent = "I'm an x-foo button!";

var XFooButton = document.registerElement('x-foo-button', {
  prototype: XFooButtonPrototype,
  extends: 'button'

Using a custom element

After registration, you can construct an instance of your element just like standard DOM elements:


If you've used extends to create a custom element that derives from an existing DOM element (e.g. something other than HTMLElement), use the is syntax:

<button is="x-foo-button"></button>

In the declarative and document.registerElement() example above, XFoo was defined as the new element's constructor. This can also be used to create an instance:

var xFoo = new XFoo();

var xFooButton = document.createElement('button', 'x-foo-button');; // "foo() called"

Browser limitations require that we supply the constructor while you supply the prototype. Use the createdCallback to do initialization work that might otherwise be in a constructor.

Polyfill details

Getting Started

First download the webcomponents.js polyfills using Bower:

bower install --save webcomponentsjs

Include the CustomElements.js or CustomElements.min.js (minified) file in your project.

<script src="bower_components/webcomponentsjs/CustomElements.js"></script>

Alternatively, you can directly use webcomponents.js (or the minified webcomponents.min.js file) in your project.

Polyfill Notes

The custom elements polyfill handles element upgrades asynchronously. The polyfill defers upgrading elements until DOMContentsLoaded time. It does this as a performance optimization. Subsequent to the initial upgrade pass, Mutation Observers are used to discover new elements.

To know when the polyfill has finished all of its start up tasks, listen to the WebComponentsReady event on document or window.


  // hide body to prevent FOUC = 0;
  window.addEventListener('WebComponentsReady', function() {
    // show body now that everything is ready = 1;

The Custom Elements specification is still under discussion. The polyfill implements certain features in advance of the specification. In particular, the lifecycle callback methods that get called if implemented on the element prototype:

  • createdCallback() is called when a custom element is created.
  • attachedCallback() is called when a custom element is inserted into a DOM subtree.
  • detachedCallback() is called when a custom element is removed from a DOM subtree.
  • attributeChangedCallback(attributeName) is called when a custom element's attribute value has changed

createdCallback is invoked synchronously with element instantiation, the other callbacks are called asynchronously. The asynchronous callbacks generally use the MutationObserver timing model, which means they are called before layouts, paints, or other triggered events, so the developer need not worry about flashing content or other bad things happening before the callback has a chance to react to changes.

Tools & Testing

For running tests or building minified files, consult the Manual Builds guide.