Erect the Fence LeetCode solution in JavaScript

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const outerTrees = (trees) => {
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  trees.sort((x, y) => {
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    if (x[0] == y[0]) return x[1] - y[1];
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    return x[0] - y[0];
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  });
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  let lower = [],
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    upper = [];
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  for (const tree of trees) {
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    while (
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      lower.length >= 2 &&
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      cmp(lower[lower.length - 2], lower[lower.length - 1], tree) > 0
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    )
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      lower.pop();
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    while (
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      upper.length >= 2 &&
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      cmp(upper[upper.length - 2], upper[upper.length - 1], tree) < 0
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    )
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      upper.pop();
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    lower.push(tree);
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    upper.push(tree);
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  }
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  return [...new Set(lower.concat(upper))];
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};
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const cmp = (p1, p2, p3) => {
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  let [x1, y1] = p1;
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  let [x2, y2] = p2;
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  let [x3, y3] = p3;
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  return (y3 - y2) * (x2 - x1) - (y2 - y1) * (x3 - x2);
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};