Roman Numeral Converter Infinite While Loop Help

JavaScript
1

So I’m doing some review and trying to write these problems out by hand on paper and when I console.log it seems to work but I keep getting an error for infinite loop and I just can’t see how. Could someone please point out what’s wrong here.

function convertToRoman(num) {

  let result = '';
  let rest = num;
  let roman = {
    'M': 1000,
    'CM': 900,
    'D': 500,
    'CD': 400,
    'C': 100,
    'XC': 90,
    'L': 50,
    'XL':40,
    'X': 10,
    'IX': 9,
    'V': 5,
    'IV': 4,
    'I': 1
  }

  for(let key in roman) {
    while (rest > 0) {
      if (rest >= roman[key]) {
        result += key;
        rest -= roman[key];
        console.log(result);
      }
    }
  }
 return result;
}

convertToRoman(68);
2

rest is changed only inside if statement, but if is executed only if rest is bigger or equal to roman[key] and the rest is decreased by roman[key]. But if rest is always bigger, how could it be < 0?