Custom Sort String LeetCode solution in JavaScript

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var customSortString = function (order, s) {
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  const hm = new Map();
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  for (let c of s) {
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    if (!hm.has(c)) hm.set(c, 0);
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    hm.set(c, hm.get(c) + 1);
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  }
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  let op = "";
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  for (let c of order) {
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    if (hm.has(c)) {
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      op += "".padStart(hm.get(c), c);
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      hm.delete(c);
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    }
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  }
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  for (let [c, occ] of hm) {
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    op += "".padStart(hm.get(c), c);
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    hm.delete(c);
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  }
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  return op;
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};