Roman Numeral Converter

function convertToRoman(num) {
  if (typeof num == "undefined" || typeof num != "number" || num > 3999) {
    return false;
  }

  let romans = "";
  const symbols = [
    [1000, "M"],
    [500, "D"],
    [100, "C"],
    [50, "L"],
    [10, "X"],
    [5, "V"],
    [1, "I"],
  ];

  // 1000
  let aa = num;
  let n1000 = parseInt(aa / 1000);

  for (let i = 0; i < n1000; i++) {
    romans += symbols[0][1];
  }

  let subTotal1 = aa;
  let subTotal5;
  let n1;
  let n5;

  for (let i = 1; i < symbols.length; i++) {

    if (i % 2) {
      // 5, 50, 500
      subTotal5 = (subTotal1 % symbols[i - 1][0]) - symbols[i][0];
      n5 = subTotal5 >= 0 ? 1 : 0;

      for (let j = 0; j < n5; j++) {
        romans += symbols[i][1];
      }
    } else {
      // 1, 10, 100
      subTotal1 = subTotal5 < 0 ? subTotal1 % symbols[i - 1][0] : subTotal5 % symbols[i - 1][0];
      n1 = parseInt(subTotal1 / symbols[i][0]);

      if (n1 > 3 && n5 >= 1) {
        romans =
          romans.substring(0, romans.length - 1) +
          symbols[i][1] +
          symbols[i - 2][1];
      } else if (n1 > 3) {
        romans += symbols[i][1] + symbols[i - 1][1];
      } else {
        for (let k = 0; k < n1; k++) {
          romans += symbols[i][1];
        }
      }
    }
  }

  return romans;
}

console.log(convertToRoman(3999));