A simple publish/subscribe store using a WeakMap to keep the subscribers private

private.js

const createStore = (function () {
  const scope = new WeakMap

  const storeProto = {

    subscribe (prop, fn) {
      const ownScope = scope.get(this)

      ownScope.subscribers[prop] = ownScope.subscribers[prop] || []
      ownScope.subscribers[prop].push(fn)

      return this
    },

    notify (prop) {
      const ownScope = scope.get(this)

      if (ownScope.subscribers[prop]) {
        ownScope.subscribers[prop].forEach(subscriber => {
          subscriber.call(this, this[prop])
        })
      }

      return this
    },

    set (prop, val) {

      this[prop] = val
      this.notify(prop)

      return this
    }
  }

  return function createStore (data) {
    const store = Object.create(storeProto)

    Object.assign(store, data)
    scope.set(store, {subscribers: {}})

    return store
  }
})()