Javascript change property inside object within array logic

        case ActionTypes.ADD_TO_CART: {        var newOrders = state.cartItems.slice();        let updatedIem = state.cartItems.find(i => {            if (i.id === action.product.id) {                return action.product            }            else {                return null            }        });        if (updatedIem) {            var oldOrder = newOrders[action.itemIndex];            var newOrder = Object.assign({}, oldOrder);            if (action.direction == 1 && newOrder.quantity > 0) {                newOrder.qty++;                newOrder.quantity--;            }            else if (action.direction == -1 && newOrder.qty != 0 && newOrder.quantity > 0) {                newOrder.qty--;                newOrder.quantity++;            }            else null            newOrders[action.itemIndex] = newOrder;        }        else {            newOrders[action.itemIndex] = action.product        }        return {            ...state,            cartItems: newOrders,        };    }

this gives me an array(cartItems) of the object but the problem here it's back with another index undefined it's similar to [{first object}, undefined,{third object}]so I don't wanna see undefined I know that the newOrders[theIndex] doing that but I don't find the solution to that or how I should replace this logic