redux utility for creating reducers reusing the same fn for multiple actions Raw

createReducer.js

const noOp = a => a

const createReducer = initialState => {
  const storedReducers = {}

  const resolveReducer = (state, action) => (typeof state === 'undefined'
    ? initialState
    : (storedReducers[action.type] || noOp)(state, action)
  )

  const traverser = actions => ({
    reducer: reducer => {
      (Array.isArray(actions) ? actions : [actions]).forEach(action => {
        storedReducers[action] = reducer
      })
      return resolveReducer
    }
  })

  resolveReducer.actions = traverser

  return resolveReducer
}

const userReducer = createReducer({ a: 42, n: null })
  .actions(['USER_1', 'USER_2'])
  .reducer((state, { type }) => ({ ...state, n: 'a', type }))

  .actions('USER_3')
  .reducer((state, { type }) => ({ ...state, n: 'b', type }))

const initState = userReducer(undefined, {})
const state1 = userReducer(initState, { type: 'USER_1' })
const state2 = userReducer(state1, { type: 'USER_2' })
const state3 = userReducer(state2, { type: 'USER_3' })
const state4 = userReducer(state3, { type: 'USER_4' })

console.log(initState, state1, state2, state3, state4)