Create Binary Tree From Descriptions LeetCode solution in JavaScript

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var createBinaryTree = function (descriptions) {
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  let nodes = new Map(),
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    children = new Set();
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  for (let [parent, child, isLeft] of descriptions) {
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    let parentNode = nodes.get(parent) || new TreeNode(parent);
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    if (!nodes.has(parent)) nodes.set(parent, parentNode);
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    let childNode = nodes.get(child) || new TreeNode(child);
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    if (!nodes.has(child)) nodes.set(child, childNode);
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    if (isLeft) parentNode.left = childNode;
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    else parentNode.right = childNode;
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    children.add(child);
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  }
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  for (let [parent, child, isLeft] of descriptions) {
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    // a node with no parent is the root
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    if (!children.has(parent)) return nodes.get(parent);
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  }
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};