INCOMPLETE: Convert number into Roman numeral in JavaScript

convert_roman.js

(function (win,doc) {
      'use strict';

      var submit = document.getElementById('submit');

      submit.addEventListener( 'click', function(){
        var number =           document.getElementById('number'),
            roman_numeral =    document.getElementById('roman');

        roman_numeral.innerHTML = convert_to_roman(number.value);
      }, false);

      function convert_to_roman(num) {
        let roman = '',
            i = 0,
            thousands = 0,
            hundreds = 0,
            remainder = 0;

        if ( num < 1 || num > 3999 ) return false;

        if ( num > 1000 ) {
          thousands =     num / 1000;

          for( i=1; i < Math.round(thousands); i++) {
            roman += 'M';
          }

          remainder = num % Math.round(thousands) * 1000;

        }

        if ( remainder > 0 ) {

          if ( remainder > 500 ) {
            hundreds =     remainder / 500;

            for( i=1; i < Math.floor(hundreds); i++) {
              roman += 'D';
            }

            remainder = remainder % Math.round(hundreds) * 500;
          }

        }

        return roman;
      }