A document section with a header that can be expanded or collapsed

Section 1

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed molestie molestie enim porta dapibus. Phasellus dolor quam, egestas eu viverra at, porttitor in diam. Donec risus tellus, accumsan eget ipsum sed, vestibulum blandit ante. Nullam rhoncus leo nec lobortis convallis. Donec posuere tellus a nibh dignissim, rhoncus viverra neque rutrum. Suspendisse rutrum at massa vitae venenatis. Suspendisse ut risus pellentesque lacus dictum aliquet. Cras a arcu id odio molestie imperdiet.

Section 2

Pellentesque vitae eros ac nulla aliquam eleifend. Nunc ornare sollicitudin arcu id suscipit. Donec sed nisl libero. Nulla facilisi. Proin ornare feugiat molestie. Mauris velit mi, volutpat sit amet posuere quis, tristique et urna. Donec sit amet tellus magna. Aenean feugiat suscipit neque, ut porttitor diam auctor in. Sed faucibus finibus ipsum et pharetra. In hac habitasse platea dictumst. Cras facilisis justo eu lectus luctus, et interdum velit aliquet.

Section 3

Aliquam vitae nulla efficitur turpis viverra placerat. Mauris fermentum tellus vel elementum aliquet. Integer vitae arcu et mi tristique lacinia. Cras placerat ultrices velit, id interdum ipsum commodo efficitur. Maecenas maximus odio a nisi dapibus, non dapibus nisl venenatis. Morbi tristique interdum leo, non tincidunt sapien efficitur ac. Nunc hendrerit turpis eget enim rhoncus sagittis. Aenean ac euismod magna. Phasellus et posuere nisi.
Demo: ExpandableSection


ExpandableSection can be used to help organize content and tools on a page.

By default, an ExpandableSection instance will be closed. You can have it appear expanded by default by setting its opened property (or attribute) to true.


import ExpandableSection from 'elix/src/ExpandableSection.js';
const expandableSection = new ExpandableSection(); // or
const expandableSection = document.createElement('elix-expandable-section');


<script type="module" src="node_modules/elix/src/ExpandableSection.js"></script>
  <h2 slot="header">Section 1</h2>
  Lorem ipsum dolor sit amet, consectetur adipiscing elit.

ExpandableSection is appropriate when:

  • You want to present the user with an overview of the available content, tools, etc.
  • You can organize the content into sections with easily-understandable names.
  • The user may want to look at more than one expanded area at a time.

See also


Ancestry: ExpandableSection → ReactiveElement → HTMLElement

Built with mixins AttributeMarshallingMixin, OpenCloseMixin, ReactiveMixin, RenderUpdatesMixin, and ShadowTemplateMixin.

By default, this uses these elements in the following roles:

$ 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.$.foo to obtain the corresponding button in the component instance's shadow tree. The $ function is simply a shorthand for getElementById, so this.$.foo is the same as this.shadowRoot.getElementById('foo').

Type: object

Defined by ShadowTemplateMixin

close(closeResult) method

Close the component (if not already closed).

Some components like AlertDialog want to indicate why or how they were closed. To support such scenarios, you can supply a value to the optional closeResult parameter. This closeResult will be made available in the whenClosed promise and the state.closeResult member.


  • closeResult: objectan indication of how or why the element closed

Defined by OpenCloseMixin

closed property

True if the element is currently closed.

Type: boolean

Defined by OpenCloseMixin

closed event

Raised when the component closes.

Defined by OpenCloseMixin

closeFinished property

True if the element has completely closed.

For components not using asynchronous open/close effects, this property returns the same value as the closed property. For elements that have a true value of state.openCloseEffects (e.g., elements using TransitionEffectMixin), this property returns true only if state.effect is "close" and state.effectPhase is "after".

Type: boolean

Defined by OpenCloseMixin

defaultState property

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

Type: object

Defined by ReactiveMixin

headerRole property

The class, tag, or template used to create the clickable header.

Type: function|string|HTMLTemplateElement

Default: SeamlessButton

open() method

Open the element (if not already opened).

Defined by OpenCloseMixin

opened event

Raised when the component opens.

Defined by OpenCloseMixin

opened property

True if the element is currently opened.

Type: boolean

Defined by OpenCloseMixin

opened-changed event

Raised when the opened/closed state of the component changes.

Defined by OpenCloseMixin

panelRole property

The class, tag, or template used to create the expandable panel.

Type: function|string|HTMLTemplateElement

Default: ExpandablePanel

refineState(state) method

Apply changes to a proposed new state for the component to enforce necessary consistency between state members. See Refining state for details.


  • state: objecta proposed new state for the component

Returns: boolean - true if the state is already acceptable as it is

Defined by ReactiveMixin

render() method

Render the component to the DOM.

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

This method invokes all internal render methods. It then invoked componentDidMount (for first render) or componentDidUpdate (for subsequent renders).

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: objectthe changes to apply to the element's state

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

Defined by ReactiveMixin

shouldComponentUpdate(nextState) method

Return true if the component should update.

The default implementation does a shallow check of property values like React's PureComponent. This seems adequate for most web components. You can override this to always return true (like React's base Component class), or to perform more specific, deeper checks for changes in state.


  • nextState: objectthe proposed new state for the element

Returns: boolean - true if the component should update (rerender)

Defined by ReactiveMixin

state property

The component's current state. The returned state object is immutable. To update it, invoke setState.

Type: object

Defined by ReactiveMixin

toggle(opened) method

Toggle the open/close state of the element.


  • opened: booleantrue if the element should be opened, false if closed.

Defined by OpenCloseMixin

updates property

The attributes and properies that should be applied to the component on render. By default, this is an empty plain object. Your mixin or component can extend this to identify the properties to set on the host element or elements in the shadow subtree.

Type: object

Defined by RenderUpdatesMixin

whenClosed() method

This method can be used as an alternative to listening to the "opened-changed" event, particularly in situations where you want to only handle the next time the component is closed.

Returns: Promise A promise that resolves when the element has completely closed, including the completion of any asynchronous opening effect.

Defined by OpenCloseMixin