Count Hills and Valleys in an Array LeetCode solution in JavaScript

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var countHillValley = function (nums) {
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  let previous;
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  let count = 0;
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  for (let i = 0; i < nums.length; i++) {
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    if (previous === undefined) {
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      previous = i;
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      continue;
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    }
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    if (nums[i - 1] === nums[i]) {
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      continue;
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    }
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    let nextCheck = i + 1;
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    while (nums[nextCheck] === nums[i]) {
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      nextCheck++;
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    }
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    if (nums[i] > nums[previous] && nums[i] > nums[nextCheck]) {
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      count++;
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    }
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    if (nums[i] < nums[previous] && nums[i] < nums[nextCheck]) {
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      count++;
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    }
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    previous = i;
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    i = nextCheck - 1;
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  }
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  return count;
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};