Purpose: lets a list-like component obtain its list items from its DOM content. It expects the component to identify its HTML "contents", and then impose some restrictions on those contents to determine which of those HTML elements are actually interesting to use as the items in the list. This mixin also allows a component to render updates to its list items.

This mixin primarily works in the updates and render phase of the Elix render pipeline:

events → methods → setState → updatesrender DOM → post-render

Expects the component to provide:

  • state.content member containing a flattened array of Node elements. You can use SlotContentMixin for this purpose.

Provides the component with:

  • itemCalcs method that can be used to calculate per-item conditions during rendering.
  • itemsForState method that calculates the items that would exist in a given state.
  • itemUpdates method to indicate what updates should be applied to a specific item.
  • Internal symbols.render method that will be invoked when the component is rendering. This is designed to interoperate with ReactiveMixin. When the render method is called, ContentItemsMixin will asks the component for itemUpdates, then applies those to the items.


import ContentItemsMixin from 'elix/src/ContentItemsMixin.js';
class MyElement extends ContentItemsMixin(HTMLElement) {}

Items differ from raw element contents in several ways:

  • They are often referenced via index.
  • They may have a selection state.
  • It's common to do work to initialize the appearance or state of a new item.
  • Text nodes (which are often whitespace) are ignored.
  • Invisible child elements are filtered out and not counted as items. Instances of invisible Node subclasses such as Comment and ProcessingInstruction are filtered out, as are invisible auxiliary elements include link, script, style, and template elements. This filtering ensures that those auxiliary elements can be used in markup inside of a list without being treated as list items.

This mixin is designed to address the Gold Standard checklist critera for Child Independence (Can you use the component with a wide range of child element types?) and Auxiliary Content (Does the component permit the use of child elements that perform auxiliary functions?). To meet those criteria, it is better for a component to filter out what it doesn't want using ContentItemsMixin than to exclusively filter _in_ only a specific type of element (using, say, querySelectorAll).


    div {
      color: gray;

If this element uses ContentItemsMixin, its items property will return the three div elements, and filter out the whitespace text nodes and the invisible style element.

Obtaining items from content

ContentItemsMixin expects a component to define a state member state.content containing the raw set of HTML elements the component contains. How a component interprets "contains" can vary, but a common definition would be the component's light DOM children. In that common case, SlotContentMixin can be used to define state.content as the light DOM children assigned to the component's default slot.

Other definitions of content are possible. Your component could, for example, define state.content to only extract nodes assigned to a particular named slot, or the children of a node in the component's shadow tree, or a set of nodes not actually in the DOM.

ContentItemsMixin defines a property called items that filters the results of state.content. This is an array of items designed for use with SingleSelectionMixin and its companion mixins. ContentItemsMixin uses the helper function content.substantiveElements to subtract out any nodes in state.content that would not normally be visible to the user.

To avoid having to recalculate the set of items each time that property is requested, the items property getter uses an object identity check on state.content. If the object referenced by state.content is the same as it was in a prior items request, then items immediately returns the same results as before.

Rendering items

List-like components often need to update their light DOM content too, e.g., to add attributes or classes to the nodes for list items. To support this need, ContentItemsMixin includes a facility for rendering state to individual list items.

When symbols.render is invoked (typically via ReactiveMixin), ContentItemsMixin will loop through the current array of nodes in items:

  • The mixin first invokes itemCalcs to give the component a chance to compute facts about the item which can be derived from component state.
  • The mixin then invokes itemUpdates, passing in the item, the calculated results of the above step, and the set of original HTML attributes on the node. The component determines what updates should be applied to that particular item, using the same updates format that RenderUpdatesMixin uses to update the top-level component.
  • Finally, the mixin applies the indicated updates to the item in question.

This process is repeated for each item in the list.

Example: The AriaListMixin updates an item's aria-selected attribute to reflect the item's selection state. It does this with code similar to:

itemUpdates(item, calcs, original) {
  const base = super.itemUpdates ? super.itemUpdates(item, calcs, original) : {};
  return merge(base, {
    attributes: {
      'aria-selected': calcs.selected

For this code to work, AriaListMixin expects to be used in conjunction with SingleSelectionMixin, which will define calcs.selected to be true for the selected item, and false for all other items.


Used by class SlotItemsMixin.

itemCalcs(item, index) method

Returns a set of calculations about the given item that can be derived from the component's current state.

The goal of the itemCalcs step is to ensure that all mixins/classes use a consistent definition for facts about an item that can be derived from component state. By default, itemCalcs includes a member index containing the index of the indicated item. Other mixins/classes can extend the result of itemCalcs to include additional facts.

For example, the SingleSelectionMixin tracks selection at the component level through a state member state.selectedIndex. When rendering a specific item, a component generally wants to know, "Is this specific item the one which is selected?". SingleSelectionMixin does this with a defintion for itemCalcs that looks like this:

itemCalcs(item, index) {
  const base = super.itemCalcs ? super.itemCalcs(item, index) : null;
  return Object.assign({}, base, {
    selected: index === this.selectedIndex

This ensures that any other aspect of the component that wants to inspect the selected state of a given item uses a consistent definition for selection.

  • item: Elementthe item being considered
  • index: numberthe item's position in the list

items property

The current set of items drawn from the element's current state.

itemUpdates(item, calcs, original) method

Determine what updates should be applied to an item to reflect the current state, using the format defined by the updates helpers.

By default, this returns an empty object. You should override this method (or use mixins that override this method) to indicate what updates should be applied to the given item during rendering.

Example: AriaListMixin uses code similar to the following to have an item's aria-selected attribute reflect its selection state:

itemUpdates(item, calcs, original) {
  const base = super.itemUpdates ? super.itemUpdates(item, calcs, original) : {};
  return merge(base, {
    attributes: {
      'aria-selected': calcs.selected

This code fragment is intended for use with SingleSelectionMixin, which provides the calcs.selected member.

  • item: Elementthe item to be updated
  • calcs: objectper-item calculations derived from element state
  • original: objectthe item's original HTML attributes, classes, and style

Returns: object the DOM updates that should be applied to the item