I'm building a shop system, so when a customer orders some items and tries to save them to the database, I want to subtract the preview product qty from the new qty ordered.

My API is sending that cart items

const cartItems = [
  {
    _id: 1,
    name: 'item one',
    price: 12.99,
    qty: 6,
  },
  {
    _id: 3,
    name: 'item three',
    price: 9.99,
    qty: 10,
  },
  {
    _id: 2,
    name: 'item two',
    price: 6.99,
    qty: 2,
  },
]

And my MongoDB return this value, so how do I substract stock number from qty?

const products =  await Product.find({})
[
    {
        _id: 1,
        name: 'item one',
        price: 12.99,
        stock: 14,
      },
      {
        _id: 2,
        name: 'item two',
        price: 6.99,
        stock: 78,
      },
      {
        _id: 3,
        name: 'item three',
        price: 9.99,
        stock: 54,
      },
]

I want to get a result like this:

[
    {
        _id: 1,
        name: 'item one',
        price: 12.99,
        stock: 8,
      },
      {
        _id: 2,
        name: 'item two',
        price: 6.99,
        stock: 76,
      },
      {
        _id: 3,
        name: 'item three',
        price: 9.99,
        stock: 44,
      },
]

CodePudding user response：

Seems a naive double nested loop could be sufficient for iterating both arrays and computing a new products array.

const newProducts = products.map((product) => ({
  ...product,
  stock:
    product.stock -
    (cartItems.find((item) => item._id === product._id)?.qty || 0)
}));

const cartItems = [
  {
    _id: 1,
    name: "item one",
    price: 12.99,
    qty: 6
  },
  {
    _id: 3,
    name: "item three",
    price: 9.99,
    qty: 10
  },
  {
    _id: 2,
    name: "item two",
    price: 6.99,
    qty: 2
  }
];

const products = [
  {
    _id: 1,
    name: "item one",
    price: 12.99,
    stock: 14
  },
  {
    _id: 2,
    name: "item two",
    price: 6.99,
    stock: 78
  },
  {
    _id: 3,
    name: "item three",
    price: 9.99,
    stock: 54
  }
];

const newProducts = products.map((product) => ({
  ...product,
  stock:
    product.stock -
    (cartItems.find((item) => item._id === product._id)?.qty || 0)
}));

console.log(newProducts);

A more optimal method would be to precompute a map of quantities by id for O(1) lookups for an overall O(n) runtime complexity (as opposed to the O(n^2) of the double-nested loop above).

const quantityMap = cartItems.reduce(
  (quantities, { _id, qty }) => ({
    ...quantities,
    [_id]: qty || 0
  }),
  {}
);

const newProducts = products.map((product) => ({
  ...product,
  stock: product.stock - (quantityMap[product._id] || 0)
}));

const cartItems = [
  {
    _id: 1,
    name: "item one",
    price: 12.99,
    qty: 6
  },
  {
    _id: 3,
    name: "item three",
    price: 9.99,
    qty: 10
  },
  {
    _id: 2,
    name: "item two",
    price: 6.99,
    qty: 2
  }
];

const products = [
  {
    _id: 1,
    name: "item one",
    price: 12.99,
    stock: 14
  },
  {
    _id: 2,
    name: "item two",
    price: 6.99,
    stock: 78
  },
  {
    _id: 3,
    name: "item three",
    price: 9.99,
    stock: 54
  }
];

const quantityMap = cartItems.reduce(
  (quantities, { _id, qty }) => ({
    ...quantities,
    [_id]: qty || 0
  }),
  {}
);

const newProducts = products.map((product) => ({
  ...product,
  stock: product.stock - (quantityMap[product._id] || 0)
}));

console.log(newProducts);