What's the difference?

script.js

var Person = function (name, yearOfBirth, job) {
    this.name = name;
    this.yearOfBirth = yearOfBirth;
    this.job = job;
  
    //This
    this.calculateAge = function () {
        console.log(2018 - this.yearOfBirth);
    }
}

//versus doing this?
Person.prototype.calculateAge = function() {
    console.log(2018 - this.yearOfBirth);
};