Add or Update product

app.js

let products = [
  {
    id: 1,
    quantity: 10,
  },
  {
    id: 2,
    quantity: 2,
  }
];

const add_product = ( products, new_item ) => {
  let is_new = true;

  products.map( item => {
    if ( item.id === new_item.id ) {
      item.quantity += new_item.quantity;

      is_new = false;
    }

    return item;
  } );

  if ( is_new ) {
    products.push( new_item );
  }

  return products;
}

// Try add existing product_id
products = add_product( products, {
  id: 2,
  quantity: 3,
} );

console.log( products );

// Try add new product_id
products = add_product( products, {
  id: 3,
  quantity: 3,
} );

console.log( products );