create-state-holder.js

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

  return {
    addListener(newListener) {
      listeners.push(newListener);
    },
    getState() {
      return state;
    },
    updateState(newState) {
      state = newState;
      listeners.forEach(listener => listener.stateChanged(newState));
    },
  };
}

const stateHolder = makeStateHolder();