how to add new property to react useContext state

Question

I have a little problem, I'm learning reactjs. I'm trying to add a property to an object in the cart array called quantity; when similar item with same id is equal to the iterator, to add a property quantity and increase it's value. I'm learning both react and javascript I'm coming from a python background so... Here is my code.

 const addToCart = (item) => {
    if (cart.length){
        for (let iter of cart){
            if (iter.id===item.id){
                item.quantity += 1 // this is 'NaN' if quantity wasn't declared.
                setCart([...cart, item])
            }else{
                setCart([...cart, item])
            }
        }
    }
}

this code does... Nothing!

Answer 1

If I understand you correctly you want to increment the quantity if the item exist in the cart, otherwise add it to the cart ? if so try the following:

const [cart, setCart] = useState([]);

...

const addToCart = (item) => {
    // declare the quantity property if it doesn't exist
    item.quantity = item.quantity || 1;
    // get the index from the cart
    const i = cart.findIndex(obj => obj.id === item.id);
    // if the item exists increment the quantity
    if(i > -1) {
      const newCart = [...cart];
      newCart[i].quantity = item.quantity + 1; 
      setCart(newCart)
    } else {
      // the item doesn't exist so add it to the cart
      setCart([...cart, item]);
    }
 }