How can I half the array

I am learning to code and I am trying out this javascript object method course. I am currently stuck on this method. I want to have the array with three different numbers(10,20,5) to be /2. I do not understand why it is returning NaN. Thank you for reading.

shirtPrice = 10
jeanPrice = 20
shoePrice = 5

shoppingList = [shirtPrice,jeanPrice,shoePrice];

const shoppingDiscountDay = {
  discountedItems: {
    calculateItemsDiscount() {
        return shoppingList/2; //return NaN
    },
  }
}
console.log(shoppingDiscountDay.discountedItems.calculateItemsDiscount());