JavaScript to convert numbers between 1 and 1,999,999 to roman numerals

romanise.js

function romanise (number, upper=true) {
  if (number > 1999999) {
    console.warn('WARN: attempted to convert number over 1,999,999 to roman numeral');
    return number;
  }
  const letters = upper ? [
    ['X', 'V', 'I'],
    ['C', 'L', 'X'],
    ['M', 'D', 'C'],
    ['X̅', 'V̅', 'M'],
    ['C̅', 'L̅', 'X̅'],
    ['M̅', 'D̅', 'C̅']
  ] : [
    ['x', 'v', 'i'],
    ['c', 'l', 'x'],
    ['m', 'd', 'c'],
    ['x̅', 'v̅', 'm'],
    ['c̅', 'l̅', 'x̅'],
    ['m̅', 'd̅', 'c̅']
  ];
  function toText (n, X, V, I) {
    let out = '';
    let r = n % 10;
    if (n > 0 && r === 0) out = X;
    else if (r <= 3) for (i=0; i<r; i++) out += I;
    else if (r < 5) out += I + V;
    else if (r <= 8) {
      out = V;
      for (i=0; i<(r-5); i++) out += I;
    } else {
      for (i=0; i<(10-r); i++) out += I;
      out += X;
    }
    return out;
  }
  function recurse (q, depth) {
    let out = '';
    let remainder = q % 10;
    let quotient = (q - remainder) / 10;
    out += toText(remainder, ...letters[depth]);
    if (quotient > 0) out = recurse(quotient, depth + 1) + out;
    return out;
  }
  return recurse(number,0);
}