The Vanilla Redux Way

0_consts.js

const INCREASE_BY = 'INCREASE_BY'

1_actions.js

import { INCREASE_BY } from './consts'

export const increaseBy = amount => ({
  type: INCREASE_BY,
  payload: amount
})

2_reducer.js

import { INCREASE_BY } from './consts'
import { increaseBy } from './actions'

const defaultState = 100
export default countReducer = (state = defaultState, action) => {
  const { payload, type } = action

  switch(type){
    case INCREASE_BY: 
      return state + payload
    default: 
      return state
  }
}