A promises version of our stateHolder

create-state-holder-P.js

function makeStateHolder() {
  let state = undefined;
  const listeners = [];

  return {
    addListener(newListener) {
      listeners.push(newListener);
    },
    getState() {
      return state;
    },
    updateState(newState) {
      state = newState;
      // Here we use a JavaScript promise to
      // call listener.stateChanged asynchronously
      listeners.forEach(listener => {
        Promise.resolve(listener).then(eventualListener => {
          eventualListener.stateChanged(newState);
        });
      });
    },
  };
}