Age Calculator - JS CSS HTML

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  Johnny Gérard•880

  @johnnygerard

  Posted over 1 year ago

  The simplest way to get the age is to compute a date difference between today and the birthdate.

  Let me give you an example:

  Pretend today is 2024-01-23 and the user's birthdate is 2000-02-28.

  Compute the year difference: 2024 - 2000 = 24. It will never be negative as long as the birthdate is in the past.

  Compute the month difference: 1 - 2 = -1. Because we have a negative value, we need to borrow a year and add 12 months.

  Currently the computed age is: 23 years, 11 months and ? days.

  Let us now compute the day difference: 23 - 28 = -5. Again a negative value, let us borrow a month and add the number of days in the previous month (December 2023).

  When borrowing a month, we need to check for a negative value and borrow a year and add 12 months again in that case.

  To get the number of days in the previous month, we can use this JavaScript expression: new Date(2024, 0, 0).getDate(). Note that the second argument (month) is zero-based and the third argument (day) is one-based. By using 0 instead of 1 for the third argument, we get the last day of the previous month instead of the first day of the current month.

  We can now add 31 to -5 which is 26. The computed age is 23 years, 10 months and 26 days.

  We can confirm this by adding the values to the birthdate.

  Birthdate: 2000-02-28

  +23 years: 2023-02-28

  +10 months: 2023-12-28

  +26 days: 2023-12-54

  We can evaluate new Date(2023, 11, 54).toDateString() which gives 'Tue Jan 23 2024'.

  Here my TypeScript code for this:

  static computeAge(year: number, month: number, day: number): Age {
      const today = new Date;
      const currentYear = today.getFullYear();
      const currentMonth = today.getMonth();
      const currentDay = today.getDate();
  
      let years = currentYear - year;
      let months = currentMonth - month;
      let days = currentDay - day;
  
      if (months < 0) {
        // Borrow a year
        years--;
        months += 12;
      }
  
      if (days < 0) {
        // Borrow a month
        months--;
        days += this.getPreviousMonthDays(currentYear, currentMonth);
  
        if (months < 0) {
          // Borrow a year
          years--;
          months += 12;
        }
      }
  
      return { years, months, days };
    }
  
    /**
   * @param month Zero-based month
   * @param year Year
   * @returns Total number of days in the previous month
   */
    private static getPreviousMonthDays(year: number, month: number): number {
      // Day 0 is shifted to the previous month's last day
      return new Date(year, month, 0).getDate();
    }
  

  Age calculator app solution

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