CustomStateSet

The CustomStateSet interface of the Document_Object_Model stores a list of possible states for a custom element to be in, and allows states to be added and removed from the set.

An HTML form element, such as a checkbox has different states, "checked" and "unchecked". Likewise, developers creating custom elements need to assign possible states to these elements. The CustomStateList allows these states to be stored, and accessed from the custom element.

A instance of CustomStateList is returned by ElementInternals.states. The CustomStateList object is described as setlike, and therefore the methods behave in a similar manner to those on a Set.

Each value stored in a CustomStateList is a <dashed-ident>, in the format --mystate.

States are stored as a <dashed-ident> as this format can then be accessed from CSS using the custom state pseudo-class. In the same way that you can use CSS to determine if a checkbox is checked using the :checked pseudo-class, you can use a custom state pseudo-class to select a custom element that is in a certain state.

CustomStateSet.size

Returns the number of values in the CustomStateSet.

CustomStateSet.add()

Adds a value to the set, first checking that the value is a <dashed-ident>.

CustomStateSet.clear()

Removes all elements from the CustomStateSet object.

CustomStateSet.delete()

Removes one value from the CustomStateSet object.

CustomStateSet.entries()

Returns a new iterator with the values for each element in the CustomStateSet in insertion order.

CustomStateSet.forEach()

Executes a provided function for each value in the CustomStateSet object.

CustomStateSet.has()

Returns a Boolean asserting whether an element is present with the given value.

CustomStateSet.keys()

An alias for CustomStateSet.values().

CustomStateSet.values()

Returns a new iterator object that yields the values for each element in the CustomStateSet object in insertion order.

The following function adds and removes the state --checked to a CustomStateSet, then prints to the console true or false as the custom checkbox is checked or unchecked.

The state of the element can be accessed from CSS using the custom state pseudo-class --checked.

set checked(flag) {
  if (flag) {
    this._internals.states.add('--checked');
  } else {
    this._internals.states.delete('--checked');
  }

  console.log(this._internals.states.has('--checked'));
}

labeled-checkbox { border: dashed red; }
labeled-checkbox:--checked { border: solid; }