The count isn't increased

Why is 'i' not increased? What's the problem? It outputs '0' in console

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<!-- language: lang-js -->

function digitalRoot(n) {
  i = 0;
  arr = [];
  let length = n.length;
  while (i < length) {
    arr.push(n[i]);
    i++;
  }
  console.log(i); // output: 0
}

digitalRoot(11);

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Here is a digitalRoot calculation

It will loop until the sum of the digits in the input has the length 1

>The digital root (also repeated digital sum) of a natural number in a given radix is the (single digit) value obtained by an iterative process of summing digits, on each iteration using the result from the previous iteration to compute a digit sum. The process continues until a single-digit number is reached. For example, in base 10, the digital root of the number 12345 is 6 because the sum of the digits in the number is 1 + 2 + 3 + 4 + 5 = 15, then the addition process is repeated again for the resulting number 15, so that the sum of 1 + 5 equals 6, which is the digital root of that number. <sub><a href="https://en.wikipedia.org/wiki/Digital_root">Wikipedia</a><sub>

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const digitalRoot = num => {
  let len = num.toString().length;
  while (len > 1) {
    num = [...num.toString()].reduce((a,b) => (+a) + (+b));
    len = num.toString().length;
  }
  return num
};  

console.log(digitalRoot(11)); // 2
console.log(digitalRoot(1999)); // 1

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>In base 10, this is equivalent to taking the remainder upon division by 9 (except when the digital root is 9, where the remainder upon division by 9 will be 0), which allows it to be used as a divisibility rule.

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<!-- language: lang-js -->

const digitalRoot = num => {
  if (num === 0) return 0;
  if (num % 9 === 0) return 9;
  return num % 9;
};

console.log(digitalRoot(11)); // 2
console.log(digitalRoot(1999)); // 1

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If you want the multiplicative digitalRoot you can do this

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const multiplicativeDigitalRoot = num => {
  let persistence = 0;
  while (num > 9) {
    num = [...num.toString()].reduce((a, b) => a * b);
    console.log(num)
    persistence++;
  }
  return { root: num, persistence }; // second number is how many iterations it took
};

console.log(multiplicativeDigitalRoot(11)); // 1,1
console.log(multiplicativeDigitalRoot(1999)); // 2,4
console.log(multiplicativeDigitalRoot(277777788888899)); // 0, 11

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I guess that you want to have an array of digits:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function digitalRoot(n) {
  const str = n.toString()
  i = 0;
  arr = [];
  while (i < str.length) {
    arr.push(str[i]);
    i++;
  }
  console.log(i);
  console.log(arr)
}

digitalRoot(11);

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Shorter:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function digitalRoot(n) {
  console.log([...n.toString()])
}

digitalRoot(11);

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Since you're passing a number in "digitalRoot(11);", the "n.length" will be undefined and the While is never triggered.

If you pass down an array instead, it will work just fine!