Find the Duplicate Number LeetCode solution in JavaScript

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var findDuplicate = function (nums) {
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  let dup = new Map();
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  for (let val of nums) {
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    if (dup.has(val)) {
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      dup.set(val, dup.get(val) + 1);
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      return val;
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    } else dup.set(val, 1);
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  }
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};