solution, using immutablejs

reducer.js

import { List, Map, fromJS } from 'immutable';

const initialState = Map({
  fruits: fromJS([ // this will turn this array into List of Map
    {
      "itemName": "banana",
      "price": 1.00,
      "quantityRemaining": 10
    },
    {
      "itemName": "apple",
      "price": 2.00,
      "quantityRemaining": 5
    },
    {
      "itemName": "raspberry",
      "price": 5.00,
      "quantityRemaining": 2
    },
    {
      "itemName": "kiwi",
      "price": 3.00,
      "quantityRemaining": 15
    },
    {
      "itemName": "pineapple,
      "price": 7.00,
      "quantityRemaining": 1
    },
    {
      "itemName": "strawberries",
      "price": 2.00,
      "quantityRemaining": 3
    }
  ])
});


function fruitReducer(state = initialState, action) {
  switch(action.type) {
    case 'DEDUCT':
      let ref = state;
      action.payload.forEach(
        deduction => {
          const index = ref.findIndex(item => item.itemName === deduction.itemName);
          ref = ref.updateIn(
            ['fruits',  index], 
            item => item.set('quantityRemaining', item.get('quantityRemaining') - deduction.quantityRemaining
        }
      );
      return ref;
    default: 
      return state;
  }
  
}