Binary Search Tree to Greater Sum Tree LeetCode solution in JavaScript

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var bstToGst = function (root) {
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  let sum = 0;
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  const traverse = (r = root) => {
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    if (!r) return null;
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    traverse(r.right);
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    let temp = r.val;
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    r.val += sum;
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    sum += temp;
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    traverse(r.left);
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  };
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  traverse();
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  return root;
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};