Digital Root Function returns “undefined”

I’m new to programming!

Trying to write a function that obtains the digital root of a number.

Currently struggling with the “for loop” and “if else” portion of the formula that returns “undefined”. Can you please give me any feedback and explain where my errors are specifically?

function digitalRoot(n) {
  let sum = 0;// define variable for loop calculations
  let str = n.toString(10,'')// convert number into string
  if (str.length === 1) { // test if string length is more than 1 character
    return str * 1; // if 1 character, return that string value and convert it back to a number
    }
  else {
    return 
    let strArray = parseInt(str.split(','))// if more than 1 character, split string into individual characters & convert back to numbers
    for (let i = 0; i < strArray.length; i++) { // sum up the numbers and repeat until there is just a single digit result
      sum += strArray[i];
    }  
  }
}