You are storing a function, not calling it that's why it should print you undefined. To fix it you should call it with list.calcAge().

Your code works and this.age = 2002 - this.birthYear creates the property you want, you just need to call it before using it.

const list = {
    name: 'Harry Potter',
    birthYear: 1995,

    calcAge: function () {
        this.age = 2002 - this.birthYear;
        return this.age;
    }
};
list.calcAge()
console.log(list.age);

Call your function. list.calcAge() You can print it like this console.log(list.calcAge()) You were close though!!