N-ary Tree Level Order Traversal LeetCode solution in JavaScript

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var levelOrder = function (root) {
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  if (!root) return [];
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  const Q = [[root, 0]];
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  const op = [];
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  while (Q.length) {
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    const [node, level] = Q.shift();
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    if (op.length <= level) {
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      op[level] = [];
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    }
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    op[level].push(node.val);
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    for (const child of node.children) {
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      Q.push([child, level + 1]);
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    }
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  }
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  return op;
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};