Calculate age explanation

JavaScript
2

Have a look the comments of the simplified code to see how I understand this.

What may look strange: there are several Date()-Contructors, two of them are used in your code [1], [2].

See:

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