I have a cart array. It stores my products, and, when I add the same item that already exists on the array, it only increases the quantity. It is like this(values only for example):

favs = [{
  key: id
  name: 'Product Name'
  value: 555
  quantity: 4
}, ]

I am creating a 'remove from cart' button. If I use slice, the whole object is deleted. So I've created the following function:

 const [favs, setFavs] = useState([])
 const { id } = useParams()
 const handleRemove = (index) => {
    if (cart[id].quantity > 1) {
      *some function here* <-
    } else {
    setCart([
        ...cart.slice(0, index),
        ...cart.slice(index+1)
    ])
    console.log('removeu')
  }}

What should I put inside the "if" to return quantity-- ? I've thought of mapping, but don't know how exactly