JSFiddle - React, Tailwind, and code Playground

JavaScript

function getAge(dateString) {

    var calendarArray = [];
    calendarArray[0] = "31";
    calendarArray[1] = "28";
    calendarArray[2] = "31";
    calendarArray[3] = "30";
    calendarArray[4] = "31";
    calendarArray[5] = "30";
    calendarArray[6] = "31";
    calendarArray[7] = "31";
    calendarArray[8] = "30";
    calendarArray[9] = "31";
    calendarArray[10] = "30";
    calendarArray[11] = "31";

    var today = new Date();
    var birthDate = new Date(dateString);
    var age = today.getFullYear() - birthDate.getFullYear();
    var m = today.getMonth() - birthDate.getMonth();
    var d = today.getDate() - birthDate.getDate();
    var dobAddYear = birthDate.getFullYear() + 1;
    if (dobAddYear < today.getFullYear() || age > 1) {
        if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
            age--;
            document.write(age + 'Y');
            return;
        }
        document.write(age + 'Y');
    } else {
        if (age === 0 && today.getDate() >= birthDate.getDate()) age = m + "M";
        if (age === 0 && today.getDate() < birthDate.getDate()) age = (m - 1) + "M";
       
        //document.write("Print" + age);
        if(age === 1) {
            var calMon = 12 - (birthDate.getMonth() - (today.getMonth()));
            if(calMon > 12) {
               document.write(age + 'Y');     
            } else {              
  document.write(12 - (birthDate.getMonth() - (today.getMonth() -1)) + 'M');
          
          }
         return;   
        }
        if (m === 0 && today.getDate() - birthDate.getDate()) {
            age = d + "D";
        }
        if (m === 0 && today.getDate() === birthDate.getDate()) {
            age = d + "D";
        }
        if (m === 1) {
            var previousMonth = calendarArray[today.getMonth() - 1];
            age = today.getDate() + (previousMonth - birthDate.getDate())

            if (age > calendarArray[today.getMonth() - 1]) {
                document.write(m + 'M');
        ...