implement createStore from scratch

createStore.js

const createStore = (reducer) => {
  // only this place create the one and only state
  let state
  let listeners = []
  
  const getState = () => state
  
  const dispatch = (action) => {
    // change state only by dispatching an action
    state = reducer(state, action)
    listeners.forEach(listener => listener())
  }
  
  const subscribe = (listener) => {
    listeners.push(listener)
    // a function for unsubscribing
    return () => {
      listeners = listeners.filter(l !== listener)
    }
  }
  
  // initialize state
  dispatch({})
  
  return {
    getState,
    dispatch,
    subscribe
  }
}

check Dan's tutorial video for more imformation