safe_fixed

function safeParse(n) {
  n = +n;

  if (isNaN(n)) throw new TypeError('NANUMERR');
  if (0 > n) throw new TypeError('NEGNUMERR');
  return n
}

function safeFixed(n) {
  return (safeParse(n)).toFixed('2');
}

try {
  console.log(safeFixed(3));
  console.log(safeFixed(-3.12));
  console.log(safeFixed('-3.4'));
  console.log(safeFixed(3.987643));
  // console.log(safeParseFloat(-3.4));
  // console.log(safeParseFloat('-3.4'));
  // console.log(safeParseFloat('-3,4'));
  // console.log(safeParseFloat('-3b4'));
  // console.log(safeParseFloat('#3b4'));
  console.log(safeFixed('#3b4'));
} catch (ex) {
  console.log(ex.message);
}