reducer.js

function createStore(reducer) {
 let state = reducer(null, undefined);

 return {
   dispatch: action => { state = reducer(action, state) },
   getState: () => state,
 }
}

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

store.getState() // => 0
store.dispatch('INCREMENT')
store.getState() // => 1
store.dispatch('DECREMENT')
store.getState() // => 0