Change state's array without changing the whole state (REACT / REDUX)

Question

I got a list of things to buy (for a little game i'm making with React) who is an array. It's in a state called "Market". And i have the other one, where i want to randomise all the value and stock them in another state.

This is my Market list:

let marketList = [
  {"name": 'product name', "qty": 0, "minPrice": 10, "maxPrice": 100, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 30, "maxPrice": 200, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 100, "maxPrice": 1500, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 200, "maxPrice": 4000, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 50, "maxPrice": 6000, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 1, "maxPrice": 400, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 60, "maxPrice": 3450, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 2, "maxPrice": 120, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 8, "maxPrice": 600, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 120, "maxPrice": 3200, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 35, "maxPrice": 100, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 300, "maxPrice": 12000, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false},
  {"name": 'product name', "qty": 0, "minPrice": 1, "maxPrice": 80, "currentPrice": 0, "historyPrice": [], "historyQty": [], "available": false}
];

I want to randomise all the value in it and store the randomised list in a state called "Tomorrow" to be able to gives hints about the next prices. This is my helpers to randomise the lists.

export function randomiseMarketList(list, day = 0){
  for(let i = 0; i < list.length; i++) {
    let item = list[i];
    let historyQtylength = item.historyQty.length;
    let random = randomAlgo(day);

    item.currentPrice = Math.floor(Math.random() * (item.maxPrice - item.minPrice)) + item.minPrice;
    item.qty = random;
    item.available = !(day == 0 || item.qty < 1);

    item.historyPrice.push(item.currentPrice);
    item.historyQty.push(item.qty);
  } 
  return list;
}

function randomAlgo(day) {
  let quart = Math.floor(Math.random() * 4);
  let multiple = Math.floor(Math.random() * 10);
  let maxQuart = Math.floor(Math.random() * (quart * multiple * day));
  let minQuart = Math.floor(Math.random() * (quart * multiple));
  return Math.floor(Math.random() * (maxQuart - minQuart)) + minQuart;
}

And this is my Tomorrow reducer:

import { ACTIONS } from '../utils/consts';
import { randomiseMarketList } from '../utils/helpers';

var initialState = {
  currentPlaceList: []
};

export function tomorrow(state = initialState, action = '') {
  switch (action.type) {

    case ACTIONS.BUILD_TOMORROW_LIST:
      console.log(action);
      let listToRandom = action.list.slice();
      let randomactionList = randomiseMarketList([...listToRandom], action.day);
      console.log(randomactionList);
      let newList = Object.assign({}, state, { currentPlaceList: randomactionList });
      return newList;

    default:
      return state;
  }
}

As you can see in my ACTIONS.BUILD_TOMORROW_LIST, i console log the action to check the value at first, and after i log the randomised list, who always has the same values. I don't understand here why they have the same values if i changed them.

I thought it was because the array was the absolute same and i changed the state directly (don't understand why). I passed the list as a new array built by the values and the spread operator, doesn't work either. I tried to create a copy of this array, doesn't work too.

The list in the tomorrow will be used to replace the current Market's list when we click on the "stay here" action.

handleStayHere(e) {
    const { dispatch, status, tomorrow, market } = this.props;
    dispatch( MarketActions.changeWholeList(tomorrow.currentPlaceList) );
    dispatch( StatusActions.changeDay( status.day + 1 ) );
    dispatch( TomorrowActions.buildTomorrowList([...market], status.day + 1) );
  }

The MarketActions.changeWholeList action works well, but i can't store correctly the tomorrow list.

Thank you for your help!

Answer 1

Looks like you don't change list's elements but mutate them in randomizer. This could lead to bizarre behaviour (debugged problem with this yesterday =)) ) I think it's better to map list values to new objects in randomiseMarketList, so you don't need to worry about item objects being the same and array slicing — it will be just plainly another object reference.

randomiseMarketList(list, day = 0){
  return list.map(item => {
    //it case there's another meaningful properties in item
    ...item,
    qty: randomAlgo(day),
    currentPrice: Math.floor(Math.random() * (item.maxPrice - item.minPrice)) + item.minPrice,
    available: !(day == 0 || item.qty < 1),
    historyPrice: [...item.historyPrice, item.currentPrice],
    historyQty: [...item.historyQty, item.qty],
})

And then in reducer

function tomorrow(state = initialState, action) {
  switch (action.type) {
    case ACTIONS.BUILD_TOMORROW_LIST:
      const currentPlaceList = randomiseMarketList(action.list, action.day);
      return {...state, currentPlaceList}

    default:
      return state;
  }
}

Another one, in reducer you're slicing array and then destructure it to another array — this looks like double work with no outcome.