Simple Redux Store

store.js

function createStore(reducer, preloadedState) {
  let state = preloadedState
  const idPool = [0]
  const listeners = new Map()

  function getState() {
    return state
  }

  function subscribe(listener) {
    let id = idPool.pop()
    if (!idPool.length) {
        idPool.push(id + 1)
    }
    listeners.set(id, listener)
    return function unsubscribe() {
      listeners.delete(id)
      idPool.push(id)
    }
  }

  function dispatch(action) {
    state = reducer(state, action)
    for ([_, listener] of listeners) {
        listener()
    }
  }

  dispatch({ type: '@@redux/INIT' })

  return { dispatch, subscribe, getState }
}