Why say discount is not defined?

Question

In practice 34.2 I get the error that discount is not defined, why?

function main() {
var prodID = readLine();
var price = parseInt(readLine(),10);
var discount = parseInt(readLine(),10);

var prod1= new Product(prodID, price);

console.log(prod1.prodID + " price: " + prod1.price);

prod1.changePrice(discount);

console.log(prod1.prodID + " new price: " + prod1.changePrice);
}

function Product(prodID, price) {
this.prodID = prodID;
this.price = price;
this.discount = discount;
this.changePrice = function(){
return this.price - (this.price * (this.discount / 100))
}
};

Answer 1

discount is only defined in main, not in Product. This priciple is called scope

Answer 2

You have to define discount outside the main function because it's locally defined in main and the product function has no access to this variable.