authWithStateMachine.js

import Auth from '../domain/auth';
import { AUTH } from './actionTypes';

const States = {
  PRISTINE: 'PRISTINE',
  VALID: 'VALID',
  INVALID: 'INVALID',
  SUBMITTING: 'SUBMITTING',
  SUCCESS: 'SUCCESS'
};

const initialState = {
  currentState: States.PRISTINE,
  data: {}
};

export const reducer = (state = initialState, action) => {
  switch(action.type) {
  case AUTH.UPDATE_AUTH_FIELD:
    const newData = { ...state.data, ...action.data };
      
    return {
      ...state,
      // ...
      data: newData,
      currentState: Auth.isValid(newData) ? States.VALID : States.INVALID
    };
  case AUTH.SUBMIT_SIGN_IN:
    if(state.currentState === States.INVALID) {
      return state; // makes it impossible to submit if it's invalid
    }
      
    return {
      ...state,
      // ...
      currentState: States.SUBMITTING
    };
  case AUTH.SIGN_IN_SUCCESS:
    return {
      ...state,
      // ...
      currentState: States.SUCCESS
    };
  }
  
  return state;
};