javascript work out age from date of birth

                                    function calculate_age(dob) { 
    var diff_ms = Date.now() - dob.getTime();
    var age_dt = new Date(diff_ms); 
  
    return Math.abs(age_dt.getUTCFullYear() - 1970);
}

console.log(calculate_age(new Date(1982, 11, 4)));

console.log(calculate_age(new Date(1962, 1, 1)));

                                    function Person(dob) {
  // [1] new Date(dateString)
  this.birthday = new Date(dob); // transform birthday in date-object
  
  this.calculateAge = function() {
    // diff = now (in ms) - birthday (in ms)
    // diff = age in ms
    const diff = Date.now() - this.birthday.getTime(); 
    
    // [2] new Date(value); -> value = ms since 1970
    // = do as if person was born in 1970
    // this works cause we are interested in age, not a year
    const ageDate = new Date(diff); 
    
    // check: 1989 = 1970 + 19
    console.log(ageDate.getUTCFullYear()); // 1989
    
    // age = year if person was born in 1970 (= 1989) - 1970 = 19
    return Math.abs(ageDate.getUTCFullYear() - 1970);
  };
}
var age =new Person('2000-1-1').calculateAge();
console.log(age); // 19

                                    // To calculate age:
var year_born = prompt("Please enter your date of birth:", "Type here");
var d = new Date();
var n = d.getFullYear();
function getAge(birthYear){
	var currentDate = new Date();
    var currentYear = currentDate.getFullYear();
    age = currentYear - birthYear;
    return age;
}
calculatedAge = getAge(year_born);
alert("Hello, " + "you are " + calculatedAge + " years old!");