Delete Nodes And Return Forest LeetCode solution in JavaScript

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var delNodes = function (root, to_delete) {
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  if (!root) return [];
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  to_delete = new Set(to_delete);
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  // know how to delete
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  // while deleting add new nodes to same algo
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  const ans = [];
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  const traverse = (r = root, p = null, d = 0) => {
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    if (!r) return null;
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    if (to_delete.has(r.val)) {
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      if (p != null) {
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        p[d == -1 ? "left" : "right"] = null;
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      }
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      traverse(r.left, null, 0);
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      traverse(r.right, null, 0);
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    } else {
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      if (p == null) ans.push(r);
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      traverse(r.left, r, -1);
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      traverse(r.right, r, 1);
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    }
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  };
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  traverse();
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  return ans;
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};