Binary Tree Pruning LeetCode solution in JavaScript

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/**
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 * Definition for a binary tree node.
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 * function TreeNode(val, left, right) {
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 * this.val = (val===undefined ? 0 : val)
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 * this.left = (left===undefined ? null : left)
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 * this.right = (right===undefined ? null : right)
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 * }
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 */
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/**
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 * @param {TreeNode} root
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 * @return {TreeNode}
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 */
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var pruneTree = function (root) {
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  let rec = function (node) {
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    if (node == null) return null;
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    if (node.val === 0 && node.left == null && node.right == null) {
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      return null;
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    }
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    node.left = rec(node.left);
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    node.right = rec(node.right);
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    // For updated tree structure if threre are leaf nodes with zero value present, if yes then return null otherewise return node itself.
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    if (node.val === 0 && node.left == null && node.right == null) {
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      return null;
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    }
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    return node;
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  };
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  rec(root);
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  if (root.val == 0 && root.left == null && root.right == null) return null;
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  return root;
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};