HTML Custom Elements Polyfill

Usage no npm install needed!

<script type="module">
  import onsenuiCustomElements from 'https://cdn.skypack.dev/@onsenui/custom-elements';


Custom Elements (v1) Polyfill Build Status

A polyfill for the custom elements v1 spec.


Include custom-elements.min.js at the beginning of your page, before any code that manipulates the DOM:

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


  1. Install and build
npm install
npm run build

(Or, npm i && gulp, if gulp is installed globally.)

  1. Test
npm run test

(Or, wct, if installed globally.)

Custom element reactions in the DOM and HTML specs

API which might trigger custom element reactions in the DOM and HTML specifications are marked with the CEReactions extended attribute.

Known Bugs and Limitations

  • adoptedCallback is not supported.
  • Changing an attribute of a customizable (but uncustomized) element will not cause that element to upgrade.
  • Only DOM API is patched. Notably, this excludes API from the HTML spec marked with the CEReactions extended attribute.
    • Unpatched API from the DOM spec:
      • Setters on Element for id, className, and slot.
      • DOMTokenList (element.classList)
      • NamedNodeMap (element.attributes)
      • Attr (element.attributes.getNamedItem('attr-name'))
  • The custom element reactions stack is not implemented.
    • Typically, DOM operations patched in this polyfill gather the list of elements to which a given callback would apply and then iterate that list, calling the callback on each element. This mechanism breaks down if an element's callback performs another DOM operation that manipulates an area of the tree that was captured in the outer operation's list of elements. When this happens, the callbacks from the inner DOM operation will be called before those of the outer DOM operation (typically, depending on the patch implementation), as opposed to a spec-compliant implementation where the callbacks are always run in the order they were inserted into each particular element's reaction queue.
  • Custom elements created by the UA's parser are customized as if they were upgraded, rather than constructed.
    • These elements are only learned about after they have been constructed, and typically after their descendants have been constructed. When these elements are constructed, their children are visible and editable even though they would not yet exist and manipulating them would throw in a spec-compliant implementation of custom elements!
  • The requirements for custom element constructors are not enforced.
    • These requirements are not generally enforcable in user script because of the ability to use the new operator on a custom element constructor. This means there is no way to know when a call to a constructor has begun or finished.
  • Methods of the ParentNode and ChildNode interfaces do not support DocumentFragments as arguments.
  • Your custom element constructor's prototype must have a property named constructor which is that constructor.
    • By default, for every constructable function F, F.prototype.constructor === F. If you replace the prototype of your constructor F, you must make sure that F.prototype.constructor === F remains true. Otherwise, the polyfill will not be able to create or upgrade your custom elements.

ES5 vs ES2015

The custom elements v1 spec is not compatible with ES5 style classes. This means ES2015 code compiled to ES5 will not work with a native implementation of Custom Elements.[0] While it's possible to force the custom elements polyfill to be used to workaround this issue (by setting (customElements.forcePolyfill = true; before loading the polyfill), you will not be using the UA's native implementation in that case.

Since this is not ideal, we've provided an alternative: native-shim.js. Loading this shim minimally augments the native implementation to be compatible with ES5 code. We are also working on some future refinements to this approach that will improve the implementation and automatically detect if it's needed.

[0] The spec requires that an element call the HTMLElement constructor. Typically an ES5 style class would do something like HTMLElement.call(this) to emulate super(). However, HTMLElement must be called as a constructor and not as a plain function, i.e. with Reflect.construct(HTMLElement, [], MyCEConstructor), or it will throw.

Parser-created elements in the main document

By default, the polyfill uses a MutationObserver to learn about and upgrade elements in the main document as they are parsed. This MutationObserver is attached to document synchronously when the script is run.

  • If you attach a MutationObserver earlier before loading the polyfill, that mutation observer will not see upgraded custom elements.
  • If you move a node with descendants that have not yet been inserted by the parser out of the main document, those nodes will not be noticed or upgraded (until another action would trigger an upgrade).

Note: Using polyfillWrapFlushCallback disconnects this MutationObserver.


tl;dr: The polyfill gets slower as the size of your page and number of custom element definitons increases. You can use polyfillWrapFlushCallback to prevent redundant work.

To avoid a potential memory leak, the polyfill does not maintain a list of upgrade candidates. This means that calling customElements.define causes a synchronous, full-document walk to search for elements with localNames matching the new definition. Given that this operation is potentially expensive and, if your page loads many custom element definitions before using any of them, highly redundant, an extra method is added to the CustomElementRegistry prototype - polyfillWrapFlushCallback.

polyfillWrapFlushCallback allows you to block the synchronous, full-document upgrade attempts made when calling define and perform them later. Call polyfillWrapFlushCallback with a function; the next time customElements.define is called and a full-document upgrade would happen, your function will be called instead. The only argument to your function is another function which, when called, will run the full-document upgrade attempt.

For example, if you wanted to delay upgrades until the document's ready state was 'complete', you could use the following:

customElements.polyfillWrapFlushCallback(function(flush) {
  if (document.readyState === 'complete') {
    // If the document is already complete, flush synchronously.
  } else {
    // Otherwise, wait until it is complete.
    document.addEventListener('readystatechange', function() {
      if (document.readyState === 'complete') {

Once your wrapper function is called (because the polyfill wants to upgrade the document), it will not be called again until you have triggered the full-document upgrade attempt. If multiple definitions are registered before you trigger upgrades, all of those definitions will apply when you trigger upgrades - don't call the provided function multiple times.

Promises returned by customElements.whenDefined will not resolve until a full-document upgrade attempt has been performed after the given local name has been defined.

let flush;
customElements.polyfillWrapFlushCallback(f => flush = f);

const p = customElements.whenDefined('c-e', () => console.log('c-e defined'));

customElements.define('c-e', class extends HTMLElement {});
// `p` is not yet resolved; `flush` is now a function.

flush(); // Resolves `p`.

You can't remove a callback given to polyfillWrapFlushCallback. If the condition your callback was intended to wait on is no longer important, your callback should call the given function synchronously. (See the document.readyState example above.)

Calling polyfillWrapFlushCallback disconnects the MutationObserver watching the main document. This means that you must delay until at least document.readyState !== 'loading' to be sure that all elements in the main document are found (subject to exceptions mentioned in the section above).

You can call polyfillWrapFlushCallback multiple times, each function given will automatically wrap and delay any previous wrappers:

customElements.polyfillWrapFlushCallback(function(flush) {
  console.log('added first');

customElements.polyfillWrapFlushCallback(function(flush) {
  console.log('added second');
  setTimeout(() => flush(), 1000);

customElements.define('c-e', class extends HTMLElement {});
// 'added second'
// ~1s delay
// 'added first'
// The document is walked to attempt upgrades.