Maximum 69 Number LeetCode solution in JavaScript

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var maximum69Number = function (num) {
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  let flag = true;
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  num = num
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    .toString()
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    .split("")
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    .map((x) => {
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      if (x !== "9" && flag) {
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        flag = false;
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        return "9";
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      }
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      return x;
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    });
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  return parseInt(num.join(""));
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};