Contains Duplicate II LeetCode solution in JavaScript

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/**
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 * @param {number[]} nums
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 * @param {number} k
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 * @return {boolean}
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 */
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var containsNearbyDuplicate = function (nums, k) {
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  const duplicateCheck = {};
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  let isValid = false;
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  for (var indexI = 0; indexI < nums.length; indexI++) {
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    if (duplicateCheck[nums[indexI]] > -1) {
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      if (Math.abs(duplicateCheck[nums[indexI]] - indexI) <= k) {
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        isValid = true;
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        break;
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      }
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    }
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    duplicateCheck[nums[indexI]] = indexI;
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  }
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  return isValid;
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};