Add to Array-Form of Integer LeetCode solution in JavaScript

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var addToArrayForm = function (num, k) {
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  const length = num.length;
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  let digit = 0,
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    index = length - 1;
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  while (k > 0 || digit > 0) {
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    if (index >= 0) {
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      digit = digit + num[index] + (k % 10);
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      num[index] = digit % 10;
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    } else {
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      digit = digit + (k % 10);
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      num.unshift(digit % 10);
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    }
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    digit = digit > 9 ? 1 : 0;
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    k = parseInt(k / 10);
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    index--;
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  }
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  if (digit) num.unshift(digit);
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  return num;
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};