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


<script type="module" src="node_modules/elix/define/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


Class hierarchy:

The element defines the following shadow parts:

  • header: the header that can be clicked/tapped to expand or collapse the panel, default type is Button
  • panel: contains the component's expandable/collapsible content, default type is ExpandablePanel
  • toggle: contains the icons or other element which lets the user know they, default type is UpDownToggle

booleanAttributeValue(name, value) static method

Given a string value for a named boolean attribute, return true if the value is either: a) the empty string, or b) a case-insensitive match for the name.

This is native HTML behavior; see the MDN documentation on boolean attributes for the reasoning.

Given a null value, this return false. Given a boolean value, this return the value as is.


  • name: string
  • value: string|boolean|null

Defined by AttributeMarshallingMixin

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

close event

Raised when the component closes.

Defined by OpenCloseMixin

closed property

True if the element is currently closed.

This read-only property is provided as a convenient inverse of opened.

Type: boolean

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: PlainObject

Defined by ReactiveMixin

headerPartType property

The class or tag used to create the header part - the header region the user can tap/click to expand or collapse the section.

Type: (component class constructor)|HTMLTemplateElement|string

Default: Button

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

open() method

Open the element (if not already opened).

Defined by OpenCloseMixin

open event

Raised when the component opens.

Defined by OpenCloseMixin

opened property

True if the element is currently opened.

This property can be set as a boolean attribute

Type: boolean

Default: false

Defined by OpenCloseMixin

openedchange event

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

Defined by OpenCloseMixin

panelPartType property

The class or tag used to create the panel part - the container for the expandable/collapsible content.

Type: (component class constructor)|HTMLTemplateElement|string

Default: ExpandablePanel

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

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

togglePartType property

The class or tag used to create the toggle part – the element that lets the user know they can expand/collapse the section.

Type: (component class constructor)|HTMLTemplateElement|string

Default: UpDownToggle

whenClosed() method

This method can be used as an alternative to listening to the "openedchange" 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