Count and Say LeetCode solution in JavaScript

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var countAndSay = function (n) {
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  if (n == 1) return "1";
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  let index = 1,
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    num = "1";
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  while (index < n) {
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    num = say(num);
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    index++;
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  }
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  return num;
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};
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function say(num) {
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  let prev = num[num.length - 1],
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    count = 0,
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    newNum = "";
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  for (let index = num.length - 1; index >= 0; index--) {
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    if (prev == num[index]) {
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      count++;
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    } else {
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      newNum = "" + count + prev + newNum;
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      count = 1;
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      prev = num[index];
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    }
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  }
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  if (count) {
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    newNum = "" + count + prev + newNum;
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  }
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  return newNum;
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}