@webcomponents/custom-elements/README.md

# Custom Elements (v1) Polyfill [![Build Status](https://travis-ci.org/webcomponents/custom-elements.svg?branch=master)](https://travis-ci.org/webcomponents/custom-elements)

A polyfill for the [custom elements](https://html.spec.whatwg.org/multipage/scripting.html#custom-elements)
v1 spec.

## Using

Include `custom-elements.min.js` at the beginning of your page, *before* any code that
manipulates the DOM:
```html
<script src="custom-elements.min.js"></script>
```

## Developing

1. Install and build

  ```
  npm install
  npm run build
  ```
  (Or, `npm i && gulp`, if [gulp](https://github.com/gulpjs/gulp) is installed globally.)

1. Test

  ```
  npm run test
  ```
  (Or, [`wct`](https://github.com/Polymer/web-component-tester), if installed
  globally.)

## Custom element reactions in the DOM and HTML specs

API which might trigger custom element reactions in the [DOM](https://dom.spec.whatwg.org/)
and [HTML](https://html.spec.whatwg.org/) specifications are marked with the
[`CEReactions` extended attribute](https://html.spec.whatwg.org/multipage/scripting.html#cereactions).

## 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](https://html.spec.whatwg.org/multipage/scripting.html#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](https://html.spec.whatwg.org/multipage/scripting.html#custom-element-conformance)
  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
  `DocumentFragment`s 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.
- The [`:defined` CSS pseudo-class](https://html.spec.whatwg.org/multipage/semantics-other.html#pseudo-classes)
  is not supported.

### 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](https://github.com/webcomponents/custom-elements/blob/master/src/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`.

### `customElements.polyfillWrapFlushCallback`

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 `localName`s 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:

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

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.

```javascript
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:

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

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.
```