Simple implementation of Redux

create-store.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.splice(listeners.indexOf(listener), 1)
    }
  }

  dispatch({})

  return { getState, dispatch, subscribe }
}

example-reducer.js

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