Как работает данный код?

const length = (str) => { 
  return str.length; 
} 
 
const addDigits = (num) => { 
  console.log(num); 
  console.log("*************"); 
  let sum = 0; 
  if (length(String(num)) == 1) { 
    return num; 
  } 
  while (length(String(num)) > 1) { 
    for (let i = 0; i <= length(String(num)) - 1; i++) { 
      console.log(i, length(String(num)) - 1); 
      sum = sum + Number(String(num)[i]); 
      console.log(sum); 
    } 
    num = sum; 
    sum = 0; 
  } 
  return num; 
} 
 
console.log(addDigits(38));