Почему не работает reduce по маске?

function dt(split){
  let now = new Date(1639171074046);
  let data = {
    d: split ? now.getDate() : ('0'+ now.getDate()).slice(-2),
    m: ('0'+ (now.getMonth() + 1)).slice(-2),
    y: now.getFullYear(),
    h: split ? now.getHours() : ('0'+ now.getHours()).slice(-2),
    i: ('0'+ now.getMinutes()).slice(-2),
    s: ('0'+ now.getSeconds()).slice(-2)
  };
  console.log(data);
  return [...'d.m.y h:i:s'].reduce((res, el) => {
    return res += data[el] || el;
  }, '');
}

console.log( dt(true) );
console.log( dt(false) );