The add method of the CustomStateSet interface adds an item to the CustomStateSet, after checking that the value is in the correct format.

value

A string which must be a <dashed-ident>, with the form --mystate.

Undefined.

SyntaxError DOMException

Thrown if the string is not a <dashed-ident>.

The following function adds the state --checked to a CustomStateSet.

class MyCustomElement extends HTMLElement {
  set checked(flag) {
    if (flag) {
      this._internals.states.add("--checked");
    }
  }
}