Question

Having 2 javascript Dates,

first is birthdate
and second is a date to calculate age from that date.

What should be the best way to do this.

Answer 1

Here is a way:

function getAge(d1, d2){
    d2 = d2 || new Date();
    var diff = d2.getTime() - d1.getTime();
    return Math.floor(diff / (1000 * 60 * 60 * 24 * 365.25));
}
console.log( getAge(new Date(1978, 10, 3)) );

Be careful with the month. Javascript counts them from 0.
1978, 10, 3 means the November 3th, 1978