Create Maximum Number LeetCode solution in JavaScript

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const maxNumber = (a, b, k) => {
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  let m = a.length,
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    n = b.length;
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  let res = [];
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  for (let i = Math.max(0, k - n); i <= Math.min(k, m); i++) {
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    let maxA = maxArray(a, i),
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      maxB = maxArray(b, k - i);
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    let merge = mergeArray(maxA, maxB);
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    if (merge > res) res = merge;
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  }
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  return res;
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};
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const maxArray = (a, k) => {
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  let drop = a.length - k,
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    res = [];
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  for (const x of a) {
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    while (drop > 0 && res.length && res[res.length - 1] < x) {
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      res.pop();
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      drop--;
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    }
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    res.push(x);
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  }
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  if (res.length >= k) {
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    res = res.slice(0, k);
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  } else {
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    res = res.concat(Array(k - res.length).fill(0));
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  }
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  return res;
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};
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const mergeArray = (a, b) => {
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  let res = [];
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  while (a.length + b.length) res.push(a > b ? a.shift() : b.shift());
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  return res;
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};