Find All Anagrams in a String LeetCode solution in JavaScript

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var findAnagrams = function (s, p) {
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  function compareMaps(map1, map2) {
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    var testVal;
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    if (map1.size !== map2.size) {
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      return false;
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    }
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    for (var [key, val] of map1) {
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      testVal = map2.get(key);
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      // in cases of an undefined value, make sure the key
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      // actually exists on the object so there are no false positives
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      if (testVal !== val || (testVal === undefined && !map2.has(key))) {
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        return false;
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      }
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    }
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    return true;
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  }
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  let p_map = new Map();
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  let s_map = new Map();
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  let res = [];
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  let pn = p.length;
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  let sn = s.length;
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  for (let i in p) {
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    p_map.set(p[i], p_map.get(p[i]) ? p_map.get(p[i]) + 1 : 1);
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    s_map.set(s[i], s_map.get(s[i]) ? s_map.get(s[i]) + 1 : 1);
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  }
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  let l = 0;
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  if (compareMaps(s_map, p_map)) res.push(l);
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  for (let r = pn; r < sn; r++) {
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    s_map.set(s[r], s_map.get(s[r]) ? s_map.get(s[r]) + 1 : 1);
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    s_map.set(s[l], s_map.get(s[l]) - 1);
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    if (s_map.get(s[l]) === 0) {
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      s_map.delete(s[l]);
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    }
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    if (compareMaps(s_map, p_map)) res.push(l + 1);
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    l++;
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  }
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  return res;
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};