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.
An 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.
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
.
class MyCustomElement extends HTMLElement {
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;
}