javascript中的箭头和常规功能表示法有什么区别？

Question

var rahul = {
  name: 'Rahul',
  lastName: 'Kumar',
  yearOfBirth: 1990,
  calculateAge: function() {
    return 2018 - this.yearOfBirth;
  }
}
console.log(rahul.calculateAge());
// output : 31
var rahul = {
  name: 'Rahul',
  lastName: 'Kumar',
  yearOfBirth: 1990,
  calculateAge: () => {
    return 2018 - this.yearOfBirth;
  }
}
console.log(rahul.calculateAge());
// Output : NaN