Sum on two values in an array of objects then insert into another array of objects

Question

I have an array of objects called stores:

stores = [
  {
    storeId: 1,
    city: "San Francisco",
    state: "CA",
  },
  {
    storeId: 2,
    city: "Seattle",
    state: "WA",
  },
  {
    storeId: 3,
    city: "Vancouver",
    state: "BC",
  },
  {
    storeId: 4,
    city: "Los Angeles",
    state: "CA",
  },
]

and another array of objects called items:

items = [
  {
    itemId: 1,
    cost: 10,
    price: 20,
    sold: false,
    _storeId: 1,
  },
  {
    itemId: 2,
    cost: 10,
    price: 20,
    sold: false,
    _storeId: 1,
  },
  {
    itemId: 3,
    cost: 5,
    price: 12,
    sold: true,
    _storeId: 2,
  },
  {
    itemId: 4,
    cost: 12,
    price: 20,
    sold: false,
    _storeId: 3,
  },
  {
    itemId: 5,
    cost: 2,
    price: 10,
    sold: false,
    _storeId: 4,
  },
  {
    itemId: 6,
    cost: 10,
    price: 50,
    sold: true,
    _storeId: 4,
  },
]

I want to sum the following categories by store:

TotalCost
TotalPrice

Then count the total items by store:

TotalItems

Then count the subtotal items sold by store:

SoldItems

so my final store array looks something like this:

storesUpdated = [
  {
    storeId: 1,
    city: "San Francisco",
    state: "CA",
    totalCost: 20,
    totalPrice: 40,
    countTotalItems: 2,
    countSoldItems: 0
  },
  {
    storeId: 2,
    city: "Seattle",
    state: "WA",
    totalCost: 5,
    totalPrice: 12,
    countTotalItems: 1,
    countSoldItems: 1
  },
  {
    storeId: 3,
    city: "Vancouver",
    state: "BC",
    totalCost: 12,
    totalPrice: 20,
    countTotalItems: 1,
    countSoldItems: 0
  },
  {
    storeId: 4,
    city: "Los Angeles",
    state: "CA",
    totalCost: 12,
    totalPrice: 60,
    countTotalItems: 2,
    countSoldItems: 1
  },
]

I've tried mapping over stores array but got stuck here:

const storesUpdated = stores.map((store) => {
   = {}
  items.forEach(item => {
    if (item._storeId === store.storeId) {
      return totalCost {
        'storeId' : item.storeId,

      }
    }
  })
})

Any ideas? Many thanks.

Callam · Accepted Answer · 2018-10-11 22:26:10Z

const stores = [{storeId:1,city:"San Francisco",state:"CA",},{storeId:2,city:"Seattle",state:"WA",},{storeId:3,city:"Vancouver",state:"BC",},{storeId:4,city:"Los Angeles",state:"CA",},]
const items = [{itemId:1,cost:10,price:20,sold:!1,_storeId:1,},{itemId:2,cost:10,price:20,sold:!1,_storeId:1,},{itemId:3,cost:5,price:12,sold:!0,_storeId:2,},{itemId:4,cost:12,price:20,sold:!1,_storeId:3,},{itemId:5,cost:2,price:10,sold:!1,_storeId:4,},{itemId:6,cost:10,price:50,sold:!0,_storeId:4,},]

const storesUpdated = stores.map((store) => {
  const updatedStore = { ...store,
    totalCost: 0,
    totalPrice: 0,
    countTotalItems: 0,
    countSoldItems: 0
  }
  
  items.forEach(item => {
    if (item._storeId === store.storeId) {
      updatedStore.totalCost += item.cost
      updatedStore.totalPrice += item.price
      updatedStore.countTotalItems += 1
      updatedStore.countSoldItems += item.sold ? 1 : 0
    }
  })
  
  return updatedStore
})

console.log(storesUpdated)

user3297291 · Accepted Answer · 2018-10-12 08:05:19Z

If you want to only loop over your data once, you could perform the merge in two steps:

Create the "empty" stores and index them by id in a single reduce: (one loop over stores)

const Store = (data) => ({ /* ... */ });
const storesById = storeData => storeData.reduce(
  (map, sd) => Object.assign(map, { [sd.storeId]: Store(sd) }),
  {}
);

Loop over the items and merge them in to the corresponding store: (one loop over items)

const addItemToStoreMap = (storeMap, item) => ({
  ...storeMap,
  [item._storeId]: addItemToStore(storeMap[item._storeId], item)
});

Note that creating all the new objects using the object spread syntax kind of cancels out the performance boost, but it's still nice that this is O(n + m) instead of O(n * m).

Putting it all together:

const storeData = [{storeId:1,city:"San Francisco",state:"CA",},{storeId:2,city:"Seattle",state:"WA",},{storeId:3,city:"Vancouver",state:"BC",},{storeId:4,city:"Los Angeles",state:"CA",},]
const itemData = [{itemId:1,cost:10,price:20,sold:!1,_storeId:1,},{itemId:2,cost:10,price:20,sold:!1,_storeId:1,},{itemId:3,cost:5,price:12,sold:!0,_storeId:2,},{itemId:4,cost:12,price:20,sold:!1,_storeId:3,},{itemId:5,cost:2,price:10,sold:!1,_storeId:4,},{itemId:6,cost:10,price:50,sold:!0,_storeId:4,},]

const { Store, addItemToStoreMap, storesById } = storeUtils();

// Loop over the stores *once*, loop over the items *once*
// During the second loop, add items to the right stores
const itemsByStoreId = itemData.reduce(addItemToStoreMap, storesById(storeData));

console.log(Object.values(itemsByStoreId));

function storeUtils() {

  // Our client-side model for a Store
  const Store = ({ storeId, city, state }) => ({
    storeId,
    city,
    state,
    totalCost: 0,
    totalPrice: 0,
    countTotalItems: 0,
    countSoldItems: 0
  });

  // Merge logic for adding an item to a store
  // Can be used in a reduce on a list of items seeded with a store
  const addItemToStore = (store, item) => ({
    ...store,
    totalCost: store.totalCost + item.cost,
    totalPrice: store.totalPrice + item.price,
    countTotalItems: store.countTotalItems + 1,
    countSoldItems: store.countSoldItems + item.sold
  });

  // Selects the right store for an item and returns a new
  // map with the updated store
  const addItemToStoreMap = (storeMap, item) => ({
    ...storeMap,
    [item._storeId]: addItemToStore(storeMap[item._storeId], item)
  });

  // Converts raw data to Store objects and indexes them by their id
  const storesById = storeData => storeData.reduce(
    (map, sd) => Object.assign(map, { [sd.storeId]: Store(sd) }),
    {}
  );

  return { Store, addItemToStoreMap, storesById };
};

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