Greatest Common Divisor of Strings LeetCode solution in JavaScript

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/**
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 * @param {string} str1
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 * @param {string} str2
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 * @return {string}
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 */
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var gcdOfStrings = function (str1, str2) {
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  const gcdOfNumber = (num1, num2) => {
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    while (num2 !== 0) {
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      const res = num2 % num1;
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      num1 = num2;
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      num2 = res;
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    }
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    return num1;
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  };
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  const gcd = gcdOfNumber(str1.length, str2.length);
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  const common = str1.slice(0, gcd);
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  return common.repeat(str1.length / gcd) === str1 &&
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    common.repeat(str2.length / gcd) === str2
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    ? common
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    : "";
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};