JSFiddle - React, Tailwind, and code Playground

HTML

<select id="daydropdown" name="D-DOB"></select>  
 <select id="monthdropdown" name="M-DOB"></select>
  <select id="yeardropdown" name="Y-DOB"></select>

JavaScript

var monthtext=['January','February','March','April','May','June','July','August','September','October','November','December'];
function populatedropdown(dayfield, monthfield, yearfield){
var today=new Date()
var dayfield=document.getElementById(dayfield)
var monthfield=document.getElementById(monthfield)
var yearfield=document.getElementById(yearfield)
for (var i=0; i<31; i++)
dayfield.options[i]=new Option(i+1)
dayfield.options[today.getDate()]=new Option(today.getDate(), today.getDate(), true, true) //select today's day
for (var m=0; m<12; m++)
monthfield.options[m]=new Option(monthtext[m], monthtext[m])
monthfield.options[today.getMonth()]=new Option(monthtext[today.getMonth()], monthtext[today.getMonth()], true, true) //select today's month
var thisyear=today.getFullYear()
for (var y = 0; y < 100; y++) {
    yearfield.options[y] = new Option(thisyear, thisyear)
    thisyear -= 1
}
yearfield.options[0]=new Option(today.getFullYear(), today.getFullYear(), true, true) //select today's year
}

window.onload=function(){
populatedropdown("daydropdown", "monthdropdown", "yeardropdown")
}