Create store basic implementation

createStore.js

const createStore = (reducer) => {
  let state
  let listeners = []

  const getState = () => state

  const dispatch = (action) => {
    state = reducer(state, action)
    listeners.forEach(listener => listener())
  }

  const subscribe = (listener) => {
    listeners.push(listener)
    return () => {
      listeners = listeners.filter(l => l !== listener)
    }
  }

  dispatch({})

  return { getState, dispatch, subscribe }
}

const counter = (state = 0, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1
    case 'DECREMENT':
      return state - 1
    default:
      return state
  }
}

const store = createStore(counter)

const renderToPage = () => {
  document.body.innerText = store.getState()
}

const renderToConsole = () => {
  console.log(store.getState())
}

store.subscribe(renderToPage)
renderToPage()
store.subscribe(renderToConsole)
renderToConsole()

document.addEventListener('click', () => {
  store.dispatch({ type: 'INCREMENT' })
})