Remove trailing 'c's. Equivalent to str.replace(/c*$/, ''). /c*$/ is vulnerable to REDOS. invert: Remove suffix of non-c chars instead. Default falsey.
function rtrim(str, c, invert) {
  if (str.length === 0) {
    return '';
  }

  // Length of suffix matching the invert condition.
  var suffLen = 0;

  // Step left until we fail to match the invert condition.
  while (suffLen < str.length) {
    var currChar = str.charAt(str.length - suffLen - 1);
    if (currChar === c && !invert) {
      suffLen++;
    } else if (currChar !== c && invert) {
      suffLen++;
    } else {
      break;
    }
  }

  return str.substr(0, str.length - suffLen);
}