Minimum Window Substring LeetCode solution in JavaScript

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var minWindow = function (s, t) {
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  const tf = {},
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    sf = {};
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  for (let c of t) {
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    tf[c] = (tf[c] || 0) + 1;
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  }
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  let l = 0,
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    r = 0,
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    rs = t.length;
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  let ml = -1,
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    mr = -1;
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  for (; r < s.length; r++) {
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    const c = s[r];
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    if (!tf[c]) continue;
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    const sc = sf[c] || 0;
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    sf[c] = sc + 1;
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    if (sf[c] <= tf[c]) {
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      rs--;
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    }
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    if (rs == 0) {
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      while (true) {
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        if (mr == -1 || mr - ml > r - l) {
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          [mr, ml] = [r, l];
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        }
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        const c = s[l];
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        if (!tf[c]) {
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          l++;
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        } else if (sf[c] - 1 < tf[c]) {
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          sf[c]--, l++, rs++;
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          break;
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        } else {
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          sf[c]--;
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          l++;
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        }
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      }
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    }
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  }
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  if (mr == -1) return "";
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  return s.slice(ml, mr + 1);
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};