Minimum Absolute Difference in BST LeetCode solution in JavaScript

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var getMinimumDifference = function (root) {
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  let order = inOrder(root, []);
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  let diff = Math.abs(order[1] - order[0]);
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  for (let i = 0; i < order.length; i++) {
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    for (let j = i + 1; j < order.length; j++) {
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      if (Math.abs(order[i] - order[j]) < diff)
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        diff = Math.abs(order[i] - order[j]);
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    }
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  }
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  return diff;
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};
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const inOrder = (root, order) => {
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  if (!root) return null;
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  inOrder(root.left, order);
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  order.push(root.val);
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  inOrder(root.right, order);
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  return order;
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};