observer.js

function initObservable() {
  let observableData = [];
  let observers = [];
  const updateData = newData => {
    observableData = newData;
    updateObservers()
  };
  const subscribe = obs => observers.push(obs);
  const unsubscribe = f => (observers = observers.filter(obs => obs !== f));

  const add = data => {
    updateData([
      ...observableData,
      {
        ...data,
        id: data.id || Date.now() + observableData.length
      }
    ]);
  };
  const remove = id => {
    updateData(observableData.filter(not => not.id !== id));
  };

  const updateObservers = () => observers.forEach(obs => obs(observableData));

  return {
    add,
    remove,
    subscribe,
    unsubscribe
  };
}

const {
  add,
  remove,
  subscribe,
  unsubscribe
} = initObservable();