Wraps a standard HTML element so it can be extended
Demo: This Elix AutoSizeTextarea element wraps a standard textarea


WrappedStandardElement exists to let you create custom element classes that extend the appearance and behavior of a standard HTML element.

The above demo of an Elix AutoSizeTextarea uses WrappedStandardElement to wrap a standard <textarea>. That gives it a foundation of all the properties, methods, and events of a standard <textarea>, allowing the component to provide additional behavior. In this case, AutoSizeTextarea automatically updates the element's height to accommodate its text.

Although the Custom Elements spec defines support for customized built-in elements, Apple has elected not to support them, effectively limiting their use. This means you cannot extend the behavior of a standard HTML element like <a> or <button> and still expect the result to work on all browsers that support Custom Elements.

As a partial workaround, the WrappedStandardElement class can create a class for you that wraps an instance of a standard HTML element. For example, the code below creates a class that will wrap an instance of a standard <a> element:

class WrappedA extends WrappedStandardElement.wrap('a') {
  customMethod() { ... }
customElements.define('wrapped-a', WrappedA);

An instance of the resulting class will look to the user like an instance of the standard element class it wraps. The resulting class will not be an instanceof the standard class (here, HTMLAnchorElement). Another limitation is that the resulting <wrapped-a> will not automatically pick up CSS styles for standard <a> elements. However, the resulting class can be extended. E.g., instances of the above class have a customMethod() available to them.

Any properties defined by the original standard element will be exposed on the resulting wrapper class, and calls to get or set those properties will be delegated to the wrapped element instance. Continuing the above example:

let wrapped = document.createElement('wrapped-a');
wrapped.href = 'http://example.com/';
wrapped.textContent = 'Click here';

Here, the created custom <wrapped-a> element will contain inside its shadow tree an instance of a standard <a> element. The call to set the wrapper's href property will ultimately set the href on the inner link. Moreover, the text content of the <wrapped-a> element will appear inside the inner link. The result of all this is that the user will see what looks like a normal link, just as if you had written <a href="http://example.com/">Click here</a>. However, the actual element will be an instance of your custom class, with whatever behavior you've defined for it.

Attributes set on the wrapping element will similarly be delegated to the inner element. For example, the AutoSizeTextarea demo above includes a placeholder="Type here" attribute on the wrapping <elix-auto-size-textarea> instance. (To see the placeholder, delete all text in the sample textarea.)The class delegates this placeholder attribute to the inner <textarea>. The inner <textarea> then renders the placeholder as a prompt to the user if the text area contains no text.

Wrapped elements should raise the same events as the original standard elements. E.g., if you wrap an <img> element, the wrapped result will raise the standard load event as expected.

Some elements, including <body>, <html>, and <style>, cannot be wrapped and still retain their normal behavior.

See also

If you're creating a custom button element, Button extends WrappedStandardElement and adds consistent keyboard and focus behavior.


Class hierarchy:

The element defines the following shadow parts:

  • inner: the inner standard HTML element

defaultState property

The default state for the component. This can be extended by mixins and classes to provide additional default state.

Type: PlainObject

Defined by ReactiveMixin

delegatesFocus property

Returns true if the component is delegating its focus.

A component using DelegateFocusMixin will always have this property be true unless a class takes measures to override it.

Type: boolean

Default: true

Defined by DelegateFocusMixin

extends property

The tag name of the standard HTML element extended by this class.

ids property

A convenient shortcut for looking up an element by ID in the component's Shadow DOM subtree.

Example: if component's template contains a shadow element <button id="foo">, you can use the reference this[ids].foo to obtain the corresponding button in the component instance's shadow tree. The ids property is simply a shorthand for getElementById, so this[ids].foo is the same as this[shadowRoot].getElementById('foo').

Type: object

Defined by ShadowTemplateMixin

inner property

Returns a reference to the inner standard HTML element.

Type: HTMLElement

render(changed) method

Render the indicated changes in state to the DOM.

The default implementation of this method does nothing. Override this method in your component to update your component's host element and any shadow elements to reflect the component's new state. See the rendering example.

Be sure to call super in your method implementation so that your component's base classes and mixins have a chance to perform their own render work.


  • changed: ChangedFlagsdictionary of flags indicating which state members have changed since the last render

Defined by ReactiveMixin

renderChanges() method

Render any pending component changes to the DOM.

This method does nothing if the state has not changed since the last render call.

ReactiveMixin will invoke this method following a setState call; you should not need to invoke this method yourself.

This method invokes the internal render method, then invokes the rendered method.

Defined by ReactiveMixin

rendered(changed) method

Perform any work that must happen after state changes have been rendered to the DOM.

The default implementation of this method does nothing. Override this method in your component to perform work that requires the component to be fully rendered, such as setting focus on a shadow element or inspecting the computed style of an element. If such work should result in a change in component state, you can safely call setState during the rendered method.

Be sure to call super in your method implementation so that your component's base classes and mixins have a chance to perform their own post-render work.


  • changed: ChangedFlags

Defined by ReactiveMixin

setState(changes) method

Update the component's state by merging the specified changes on top of the existing state. If the component is connected to the document, and the new state has changed, this returns a promise to asynchronously render the component. Otherwise, this returns a resolved promise.


  • changes: PlainObjectthe changes to apply to the element's state

Returns: Promise - resolves when the new state has been rendered

Defined by ReactiveMixin

state property

The component's current state.

The returned state object is immutable. To update it, invoke internal.setState.

It's extremely useful to be able to inspect component state while debugging. If you append ?elixdebug=true to a page's URL, then ReactiveMixin will conditionally expose a public state property that returns the component's state. You can then access the state in your browser's debug console.

Type: PlainObject

Defined by ReactiveMixin

stateEffects(state, changed) method

Ask the component whether a state with a set of recently-changed fields implies that additional second-order changes should be applied to that state to make it consistent.

This method is invoked during a call to internal.setState to give all of a component's mixins and classes a chance to respond to changes in state. If one mixin/class updates state that it controls, another mixin/class may want to respond by updating some other state member that it controls.

This method should return a dictionary of changes that should be applied to the state. If the dictionary object is not empty, the internal.setState method will apply the changes to the state, and invoke this stateEffects method again to determine whether there are any third-order effects that should be applied. This process repeats until all mixins/classes report that they have no additional changes to make.

See an example of how ReactiveMixin invokes the stateEffects to ensure state consistency.


  • state: PlainObjecta proposal for a new state
  • changed: ChangedFlagsthe set of fields changed in this latest proposal for the new state

Returns: PlainObject

Defined by ReactiveMixin

template property

The template copied into the shadow tree of new instances of this element.

The default value of this property is a template that includes an instance the standard element being wrapped, with a <slot> element inside that to pick up the element's light DOM content. For example, if you wrap an <a> element, then the default template will look like:

  :host {
    display: inline-block;
  <a id="inner">

The display styling applied to the host will be block for elements that are block elements by default, and inline-block (not inline) for other elements.

If you'd like the template to include other elements, then override this property and return a template of your own. The template should include an instance of the standard HTML element you are wrapping, and the ID of that element should be "inner".

Type: string|HTMLTemplateElement

wrap(extendsTag) static method

Creates a class that wraps a standard HTML element.

Note that the resulting class is a subclass of WrappedStandardElement, not the standard class being wrapped. E.g., if you call WrappedStandardElement.wrap('a'), you will get a class whose shadow tree will include an anchor element, but the class will not inherit from HTMLAnchorElement.


  • extendsTag: stringthe standard HTML element tag to extend