Permutation of a string

Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise.

In other words, return true if one of s1's permutations is the substring of s2.

Example 1:

Input: s1 = "ab", s2 = "eidbaooo"
Output: true
Explanation: s2 contains one permutation of s1 ("ba").

var checkInclusion = function (s1, s2) {
  if (s1.length > s2.length) {
    return false;
  }

  const freqMap = {};
  let count = 0;
  let start = 0;
  let end = 0;

  for (let char of s1) {
    if (freqMap[char] === undefined) count++;
    freqMap[char] = (freqMap[char] || 0) + 1;
  }

  while (s2.length > end) {
    let nextChar = s2[end];

    if (freqMap[nextChar] !== undefined) {
      freqMap[nextChar]--;
      if (freqMap[nextChar] === 0) {
        count--;
      }
    }

    end++;
    if (count === 0) {
      return true;
    }

    if (end - start === s1.length) {
      let temp = s2[start];
      if (freqMap[temp] !== undefined) {
        freqMap[temp] += 1;
        if (freqMap[temp] === 1) count++;
      }
      start++;
    }
  }
  return false;
};


console.log(checkInclusion("dc", "odicf"));