const object = {
  1234: {
    quantity: 8
  },
  5678: {
    quantity: 16
  },
  9101: {
    quantity: -2
  }
};

const [index] = Object
  .entries(object)
  .reduce(([maxKey, maxValue], [key, value]) => value.quantity >= maxValue.quantity ? [key, value] : [maxKey, maxValue]);