Tried to make the simplest store module I could muster. Works pretty well.

simple-store.mjs

var listeners = []
var state = {}
var noop = x => x

function subscribe (fn) {
  listeners.push(fn)
}

function unsubscribe (fn) {
  listeners.splice(listeners.indexOf(fn), 1)
}

function dispatch (action) {
  action = action || noop
  let l = listeners.length
  let fn
  state = action(state) || state
  for (let i = 0; i < l; i++) {
    fn = listeners[i]
    fn(state)
  }
}

function store (initialState) {
  if (initialState) {
    state = Object.assign(state, initialState)
  }
  return state
}

store.subscribe = subscribe
store.unsubscribe = unsubscribe
store.dispatch = dispatch

export default store