Why is it getting undefined after return in recursive function?

function superDigit(n, k) {
  // Write your code here
  let str = "";
  for (let i = 0; i < k; i++) {
    str = str + n;
  }
  if (Math.floor(parseInt(str) / 10) === 0) {
    return n;
  } else {
    
      let t=calculateSum(str.split(''));
     console.log('done:',t)
     return t
     
  }
  function calculateSum(value) {
    let arr = value;
    let sum = 0;
    console.log(arr);
    for (let i = 0; i < arr.length; i++) {
        sum = sum + parseInt(arr[i]);
    }
    
    if (Math.floor(sum / 10) === 0) {
      console.log(sum);
      return sum;
    } else {
      calculateSum(sum.toString().split(''));
    }
  }
}

console.log("tttt:"+superDigit(9, 3));