Custom Elements Polyfill
Switch branches/tags
Nothing to show
Clone or download
Pull request Compare This branch is 218 commits behind googlearchive:master.
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.
MutationObservers @ b4c77f3
buildbot @ e24d6ae
tools @ fa820a7

Learn the tech

Why Custom Elements?

Custom Elements let authors define their own 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. Their name must always contain a dash (-).

Element registration

Before you can use a custom element, it needs to be registered using one of the following methods. Otherwise, the browser considers it an HTMLUnknownElement.


The <element> tag provides a mechanism to encapsulate HTML, CSS, and JavaScript into reusable, encapsulated components.

<element name="x-foo" constructor="XFoo">
    I'm an x-foo!
    // When <element> is in document, we might run in wrong context.
    // Only do work when this == <element>.
    if (this !== window) {
      var section = this.querySelector('section');

      // Has built-in 'window' protection.
        prototype: {
          readyCallback: function() {
            this.innerHTML = section.innerHTML;
          foo: function() {
            console.log('foo() called');

The constructor attribute name goes on global scope and can be used to construct the element in JavaScript.

Extending existing elements

Using <element>, the prototype must be a simple object, but the extends attribute can be used to extend existing DOM elements. The system chains the correct prototype based the value of this attribute.

Example of extending button:

<element name="x-foo" extends="button">

To register a new custom element in JavaScript, invoke document.register() 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.readyCallback = function() {
  this.textContent = "I'm an x-foo!";
}; = function() {
  console.log('foo() called');

var XFoo = document.register('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.register():

Example extending button:

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

var XFoo = document.register('x-foo', {
  prototype: XFooPrototype,
  extends: 'button'

Using a custom element

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


In the declarative and document.register() examples above, XFoo was defined as the new element's constructor. Browser limitations require that we supply the constructor while you supply the prototype. Use the readyCallback to do initialization work that might otherwise be in the constructor.

var xFoo = new XFoo();

var xFoo2 = document.createElement('x-foo');; // "foo() called"

Polyfill details

Getting Started

Include the custom-elements.js or custom-elements.min.js (minified) file in your project.

<script src="CustomElements/custom-elements.js"></script>

custom-elements.js is the debug loader and uses document.write to load additional modules. Use the minified version (custom-elements.min.js) if you need to load the file dynamically.

Polyfill Notes

The polyfill parses <element> tags and handles element upgrades asynchronously. To know when the polyfill has finished all 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, there are several notification callback methods that are used if implemented on the element prototype.

  • readyCallback() is called when a custom element is created.
  • insertedCallback() is called when a custom element is inserted into a DOM subtree.
  • removedCallback() 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

readyCallback is invoked synchronously with element instantiation, the other callbacks are called asyncronously. 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.

The extends option to document.register() (discussed above) is exclusive to this polyfill.

Tools & Testing

For running tests or building minified files, consult the tooling information.