The return statement is throwing out an undefined value for a recursive function [duplicate]

I am trying to solve this question->
https://www.codewars.com/kata/541c8630095125aba6000c00/train/javascript

Basically the purpose of the function is to:
16 –> 1 + 6 = 7
942 –> 9 + 4 + 2 = 15 –> 1 + 5 = 6
132189 –> 1 + 3 + 2 + 1 + 8 + 9 = 24 –> 2 + 4 = 6
493193 –> 4 + 9 + 3 + 1 + 9 + 3 = 29 –> 2 + 9 = 11 –> 1 + 1 = 2

I am able to get the correct answer (“final sum”) in my console.log but I am not able to get that value in the written statement.

function digital_root(n) {
  console.log("initial", n)
  let sum=0
  
  while(n>0){
    
    sum+= n%10
    console.log("sum",sum)
    n= Math.floor(n/10)
    console.log("num",n)
    
  }
  if(sum>10){
    digital_root(sum)
  } else{
      console.log("final sum",sum)
      return sum}
}

Here’s the console.log->

initial 16
sum 6
num 1
sum 7
num 0
final sum 7
initial 195
sum 5
num 19
sum 14
num 1
sum 15
num 0
initial 15
sum 5
num 1
sum 6
num 0
final sum 6

initial 398754
sum 4
num 39875
sum 9
num 3987
sum 16
num 398
sum 24
num 39
sum 33
num 3
sum 36
num 0
initial 36
sum 6
num 3
sum 9
num 0
final sum 9

its returning undefined instead of 6 & 9 respectively.

Also a humble request is to not suggest an alternative solution but to help me fix this one.
Thank you