I want to apply prototype inheritance so my code should console logging 18 instead it is giving NaN

  'use strict';
    
    const Person= function(firstName,birthYear){
        this.name=firstName;
        this.year=birthYear;
    }
    
    const mahir=new Person("mahir",2003);
    console.log(mahir);
    
    // to check if mahir is instance of person
    
    console.log(mahir instanceof Person)
    
    // prototype inheritance
    
    Person.prototype.calcAge=()=>{
        console.log (2020-this.year);
    }
    mahir.calcAge();