javascriptで文字列の間に記号を入れたい

function addSignToString(number){
  const numArray = String(number).match(/.{2}/g);
  const reducer = (accumulator, currentValue) => accumulator +  ' > ' + currentValue;
  signIncludedString = numArray.reduce(reducer);
  return signIncludedString;
}

console.log(addSignToString(121212));
// 12 > 12 > 12

修正案_1

@e92gr sann
@hxiqz sann
コメントありがとうございます。

function addSignToString(number){
  const stringNum = String(number);

  if (isNullOrUndefined(number)) {
      return '';
  } else if (stringNum.length < 2 ) {
      return number;
  } else if (stringNum.length % 2 === 1) {
      const last = stringNum.substring(stringNum.length - 1, stringNum.length);
      return addSign(stringNum) + ' > ' + last;
  } else {
     return addSign(stringNum);
  }
}

function addSign(number) {
  if(!['string', 'number', 'bigint'].includes(typeof number)) return '';
  const numArray = String(number).match(/.{2}/g);
  return numArray !== null ? numArray.join(' > ') : '';
}

function isNullOrUndefined(number) {
    if (number === null | number === undefined | number === '' | !number) {
        console.log('入力がありません');
        return true;
    }
}
console.log(addSignToString(121212));
// 12 > 12 > 12
console.log(addSignToString(122));
// 12 > 2
console.log(addSignToString());
// 入力がありません