How do I pass a variable into a JS while loop?

Question

I want to recursively sum an integer: to split an integer into an array and then sum the individual items until I am left with a single integer array.

This is my logic:

1. Take an integer (n) and split it into an array.
2. Sum all the individual numbers.
3. Is the number greater than 9?
   1. YES: Repeat steps 1 and 2
      
   2. NO: Return the number

function digital_root(n) {
  let digits = (""+n).split("").map(Number);
  while (digits.length > 1) {
    let result = digits.reduce((sum, int) => sum + int);
    let digits = (""+result).split("").map(Number);
  }
  return digits;
};

This is the error code my Node.js chucks at me (at line 4 in the example code above):

ReferenceError: digits is not defined
    at digital_root (repl:6:18)

I'm assuming that the variable digits is accessible inside the scope of the while loop, but obviously, I seem to be wrong? Could someone shed some insight here for me, please?

---

EDIT: Thank you everyone for your help! I've solved the issue. For your curiosity, this is the mathematical form underlying my algorithm: http://mathworld.wolfram.com/DigitalRoot.html

It could also have been solved in one line:

function digital_root(n) {
  return (n - 1) % 9 + 1;
}

Answer 1

The inner let for digits (line 5) should be removed

function digital_root(n) {
  let digits = (""+n).split("").map(Number);
  while (digits.length > 1) {
    let result = digits.reduce((sum, int) => sum + int);
    digits = (""+result).split("").map(Number);
  }
  return digits;
}

Answer 2

Issue is let keyword, as let has block level scope, "digits" is not defined at the start of the while loop

Removing let for digits in while loop

function digital_root(n) {
  let digits = (""+n).split("").map(Number);
  while (digits.length > 1) {
    let result = digits.reduce((sum, int) => sum + int);
    digits = (""+result).split("").map(Number);
  }
  return digits;
}

console.log(digital_root(47))