Add One Row to Tree 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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 * @param {number} val
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 * @param {number} depth
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 * @return {TreeNode}
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 */
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var addOneRow = function (root, val, depth) {
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  if (depth === 1) return new TreeNode(val, root);
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  const refactor = (node = root, currentDep = 1) => {
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    if (!node) return;
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    if (currentDep === depth - 1) {
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      const { left, right } = node;
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      node.left = new TreeNode(val, left);
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      node.right = new TreeNode(val, null, right);
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    }
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    refactor(node.left, currentDep + 1);
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    refactor(node.right, currentDep + 1);
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  };
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  refactor();
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  return root;
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};