How to reduce this array of objects to this one?

Question

I need to reduce an array of objects, the reduce function should count the number of repeated objects in the array and set the number as a value of quantity property in the repeated object.

const x = {a: 1, b: 3, quantity: 1};
const y = [{a: 2, b: 5}, {a: 0, b: 5}, x, {a: 10, b: 3}, x, x, {a: 4, b: 6}]

How to reduce y to equal:

[{a: 2, b: 5}, {a: 0, b: 5}, {a: 1, b: 3, quantity: 3}, {a: 4, b: 6}]

This is my attempt:

for (let i = 0; i < y.length; i++) {
       for (let j = i + 1; j < y.length; j++) {
         if (y[i] === y[j]) {
           y[i].quantity += 1;
           y[j] = null;
         }
       }
     }

Answer 1

Basically you need to check each property of the objects without quantity for getting a condensed result.

 var x = { a: 1, b: 3, quantity: 1 }, y = [{ a: 2, b: 5 }, { a: 0, b: 5 }, x, { a: 10, b: 3 }, x, x, { a: 4, b: 6 }], result = y.reduce((r, o) => { var keys = Object.keys(o); temp = r.find(q => Object.keys(q).length === keys.length && keys.every(k => k === 'quantity' || o[k] === q[k])); if (temp) { temp.quantity = (temp.quantity || 1 ) + 1; } else { r.push(o); } return r; }, []); console.log(result); 

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Answer 2

Here is an easier approach, be careful not to mutate data.

Working example: https://stackblitz.com/edit/stackoverflow-transformation-3?file=index.js

const x = { a: 1, b: 3, quantity: 1 };
const y = [{  a: 2, b: 5 }, { a: 0, b: 5 }, x, { a: 10, b: 3 }, x, x, { a: 4, b: 6 }]

const output = y.reduce((acc, x, i) => {
  const index = acc.findIndex(y => y.hasOwnProperty('quantity'))
   // if the first obj with prop quantity is added
   // then just add the current value to the accumulator
  if (index > 0 && x.hasOwnProperty('quantity')) {
    acc[index] = {
      ...acc[index],
      quantity: acc[index]['quantity'] + x.quantity
    }
  }
  else {
    acc.push(x)
  }
  return acc;

}, [])

console.log('output', output)

UPDATE

Second approach, process of adding a product to the cart

// product which wants to be added to the cart
const selProduct = { id: 1, quantity: 1 };
// unique items by id in the car only
const cart = [
  { id: 0, quantity: 1 },
  { id: 2, quantity: 1 },
  { id: 3, quantity: 1 },
  { id: 1, quantity: 1 }
]
// check if selected product exists in the cart
const index = cart.findIndex(x => x.id === selProduct.id);
// if not, add it to the cart
if (index < 0) {
  cart.push(selProduct);
}
// if yes, increment the quantity of the already found product
else {
  cart[index] = {
    ...cart[index],
    'quantity': cart[index]['quantity'] + selProduct['quantity']
  }
}
console.log(cart)

You should never have multiple products with the same id in the cart and then preform the removal of duplicates...