Reducer is subset of Rx

rx-reducer.js

const Rx = require('rx')

const combineReducers = reducerMap => {
  const initialState = Object.entries(
    reducerMap
  ).reduce((acc, [key, reducer]) => {
    return Object.assign({}, acc, {
      [key]: reducer(undefined, { type: '__init' })
    })
  }, {})
  return (state = initialState, action) => {
    console.log('run combined reducer')
    return Object.entries(reducerMap).reduce((acc, [key, reducer]) => {
      return Object.assign({}, acc, {
        [key]: reducer(state[key], action)
      })
    }, {})
  }
}

const ADD = 'add'
const add = n => ({ type: ADD, payload: n })

const initialState = {
  value: 0
}

const counter = (state = initialState, action) => {
  switch (action.type) {
    case ADD:
      return { value: action.payload + state.value }
    default:
      return state
  }
}

const rootReducer = combineReducers({ counter })

const actions = Rx.Observable.from([add(1), add(2), add(3)])
const scanned = actions.scan(rootReducer, undefined) // need undefined to pass with reducer on init

scanned.subscribe(x => {
  console.log('subscribe', x)
})