Add or Update product (use Array.prototype.findIndex())

app.js

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

const add_product = ( products, new_item ) => {
  let index = products.findIndex( obj => obj.id == new_item.id );

  if ( index > -1 ) {
    products[index].quantity += new_item.quantity;  
  } else {
    products.push( new_item );
  }

  return products;
}

products = add_product( products, {
  id: 2,
  quantity: 3,
} );

console.log( products );

products = add_product( products, {
  id: 3,
  quantity: 3,
} );

console.log( products );