how to code a check age function in javascript

1function Person(dob) {
2  // [1] new Date(dateString)
3  this.birthday = new Date(dob); // transform birthday in date-object
4  
5  this.calculateAge = function() {
6    // diff = now (in ms) - birthday (in ms)
7    // diff = age in ms
8    const diff = Date.now() - this.birthday.getTime(); 
9    
10    // [2] new Date(value); -> value = ms since 1970
11    // = do as if person was born in 1970
12    // this works cause we are interested in age, not a year
13    const ageDate = new Date(diff); 
14    
15    // check: 1989 = 1970 + 19
16    console.log(ageDate.getUTCFullYear()); // 1989
17    
18    // age = year if person was born in 1970 (= 1989) - 1970 = 19
19    return Math.abs(ageDate.getUTCFullYear() - 1970);
20  };
21}
22var age =new Person('2000-1-1').calculateAge();
23console.log(age); // 19
24