Maximum Value at a Given Index in a Bounded Array LeetCode solution in JavaScript

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var maxValue = function (n, index, maxSum) {
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  function getSum(len, v) {
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    var sum = 0;
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    if (len <= v) {
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      sum = ((2 * v - len + 1) * len) / 2;
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    } else {
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      sum = ((1 + v) * v) / 2 + len - v;
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    }
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    return sum;
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  }
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  function isValid(mid) {
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    var curSum = getSum(leftLen, mid) + getSum(rightLen, mid) - mid;
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    return curSum <= maxSum;
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  }
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  var l = 0;
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  var r = 1000000000;
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  var leftLen = index + 1;
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  var rightLen = n - index;
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  while (l < r) {
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    var mid = Math.ceil((l + r) / 2); // handle dead loop when l===r & l=mid
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    if (!isValid(mid)) {
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      r = mid - 1;
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    } else {
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      l = mid;
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    }
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  }
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  return l;
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};