class Increment {
  constructor (by) {
    this.by = by
  }

  getNextState (count) {
    return count + this.by
  }
}

class Decrement = {
  constructor (by) {
    this.by = by
  }

  getNextState (count) {
    return count - this.by
  }
}


const reducer = (state = 0, action) => action.getNextState(state)
const store = createStore(reducer)

plusTen = new Increment(10)
minusFive = new Decrement(5)

store.dispatch(plusTen)
store.dispatch(plusTen)
store.dispatch(minusFive)

store.getState() // 15