Diameter of Binary Tree LeetCode solution in JavaScript

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function fastDiameter(node) {
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  if (node == null) {
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    let pair = new Array(2).fill(0);
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    pair[0] = 0;
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    pair[1] = 0;
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    return pair;
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  }
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  let leftPair = fastDiameter(node.left);
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  let rightPair = fastDiameter(node.right);
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  let leftDiameter = leftPair[0];
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  let rightDiameter = rightPair[0];
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  let height = leftPair[1] + rightPair[1] + 1;
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  let maxDiameter = Math.max(leftDiameter, rightDiameter, height);
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  let maxHeight = Math.max(leftPair[1], rightPair[1]) + 1;
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  return [maxDiameter, maxHeight];
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}
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// diameter --> number of edges between two end nodes
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var diameterOfBinaryTree = function (root) {
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  let pair = fastDiameter(root);
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  return pair[0] - 1;
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};