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I have make a ES6 class function, I can't understand why the console result is showing NaN

class Person 
 constructor(firstName, lastName, dob) 
 this.firstName = firstName;
 this.lastName = lastName;
 this.birthday = new Date(dob);
 

 greeting() 
 return `Hello there, This is $this.firstName $this.lastName`;
 

 calculateAge() 
 const diff = Date.now() - this.birthday.getTime();
 const ageDate = new Date(diff);
 return Math.abs(ageDate.getUTCFullYear() - 1995);
 


const niran = new Person('Niran', 'Yousuf', '26-12-1992');

console.log(niran.calculateAge());

The syntax for Date is:

new Date();
new Date(value);
new Date(dateString);
new Date(year, monthIndex [, day [, hours [, minutes [, seconds [, milliseconds]]]]]);

Also there is no such thing (without a library) as DateInterval like you can in PHP.

The problem is in the string that you are sending to the Date object on line 5. Just change the values like new Date(26,12,1992).

The syntax for Date object is :

new Date()

new Date(year, month, day, hours, minutes, seconds, milliseconds)

new Date(milliseconds)

new Date(date string)

Check out the docs at : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date

You can fix the date diff logic. The problem lies in your difference and how you are passing the date

var date1 = new Date("7/13/2010"); // this is your birth year
var date2 = new Date(); // This is your current date
var timeDiff = Math.abs(date2.getTime() - date1.getTime());
var diffDays = Math.ceil(timeDiff / (1000 * 3600 * 24 * 365)); 
alert(diffDays);

Rest is all logic to calculate date