Why is this code turning up 'NaN'?

Question

function person(name, age, Game){
this.name= name;
this.age= age;
this.favGame= Game;
this.Year= BirthYear();
}

function BirthYear() {
return 2018 - this.age;
}

var Ed = new person(Ed, 28, "Majora's Mask");
var Ann = new person(Ann, 29, "Breath of the Wild");

document.write(Ed.Year);

Answer 1

Put the BirthYear function inside the person function to make it a member.

function person(name, age, Game) {
this.name = name;
this.age = age;
this.favGame = Game;
this.Year = BirthYear();

function BirthYear() {
return parseInt(new Date().getFullYear() - age);
}
}

P.S. Please don't use too many words on question's tags, it's confusing and misleading.

Answer 2

The problem is caused by the BirthYear function.
This is because it sits outside the person object.
Specifically it is because this.age == window.age not person.age.
Also use Ipangs Date type basrd solution as this will likely work better.