Binary Tree Tilt LeetCode solution in JavaScript

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var findTilt = function (root) {
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  function helper(node, acc) {
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    if (node === null) {
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      return 0;
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    }
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    const left = helper(node.left, acc);
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    const right = helper(node.right, acc);
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    acc.sum += Math.abs(left - right);
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    return left + node.val + right;
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  }
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  let acc = { sum: 0 };
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  helper(root, acc);
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  return acc.sum;
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};