Base 7 LeetCode solution in JavaScript

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/**
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 * @param {number} num
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 * @return {string}
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 */
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var convertToBase7 = function (num) {
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  let str = "",
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    sign;
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  if (num < 0) {
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    sign = -1;
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    num *= -1;
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  }
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  while (num >= 7) {
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    str += num % 7;
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    num = Math.floor(num / 7);
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  }
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  str += num;
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  if (sign < 0) str += "-";
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  return str.split("").reverse().join("");
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};