I don't know why this 'this' method doesn't work?

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);

Try this. It will work.

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

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

console.log(list.calcAge());

Try calling list.calcAge() in your console

the calcAge is function property so you must be treated as a function list.calcAge()

const list = {
name: 'Harry Potter',
birthYear: 1995,
calcAge: function () {
  this.age = 2002 - this.birthYear;
  return this.age;
    }
};
document.getElementById("name").innerHTML = list.name;
document.getElementById("birthYear").innerHTML = list.birthYear;
document.getElementById("age").innerHTML = list.calcAge();

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