Find First and Last Position of Element in Sorted Array LeetCode solution in JavaScript

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var searchRange = function (nums, target) {
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  if (nums.length === 1) return nums[0] === target ? [0, 0] : [-1, -1];
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  const findFirstInstance = (left, right) => {
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    if (left === right) return left;
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    var pivot;
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    while (left < right) {
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      if (left === pivot) left += 1;
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      pivot = Math.floor((left + right) / 2);
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      if (nums[pivot] === target) {
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        if (nums[pivot - 1] !== target) return pivot;
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        return findFirstInstance(left, pivot - 1);
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      }
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      if (nums[pivot] > target) right = pivot;
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      else left = pivot;
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    }
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  };
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  const findLastInstance = (left, right) => {
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    if (left === right) return left;
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    var pivot;
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    while (left < right) {
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      if (left === pivot) left += 1;
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      pivot = Math.floor((left + right) / 2);
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      if (nums[pivot] === target) {
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        if (nums[pivot + 1] !== target) return pivot;
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        return findLastInstance(pivot + 1, right);
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      }
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      if (nums[pivot] > target) right = pivot;
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      else left = pivot;
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    }
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  };
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  var left = 0,
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    right = nums.length - 1,
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    pivot;
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  while (left < right) {
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    if (left === pivot) left += 1;
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    pivot = Math.floor((left + right) / 2);
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    if (nums[pivot] === target) {
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      var first =
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        nums[pivot - 1] !== target ? pivot : findFirstInstance(left, pivot - 1);
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      var last =
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        nums[pivot + 1] !== target ? pivot : findLastInstance(pivot + 1, right);
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      return [first, last];
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    }
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    if (nums[pivot] > target) right = pivot;
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    else left = pivot;
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  }
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  return [-1, -1];
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};