Permutation in String

Problem statement

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: trueExplanation: s2 contains one permutation of s1 ("ba").

Example 2:

Input: s1 = "ab", s2 = "eidboaoo"Output: false

Constraints:

• 1 <= s1.length, s2.length <= 104
• s1 and s2 consist of lowercase English letters.

My solution

/**
 * @param {string} s1
 * @param {string} s2
 * @return {boolean}
 */
var checkInclusion = function(s1, s2) {
    const patternMap = new Map();
    const stringMap = new Map();
    
    for (let i = 0; i < s1.length; i++) {
        const s1Char = s1.charAt(i)
        const s2Char = s2.charAt(i)
        
        patternMap.set(s1Char, (patternMap.get(s1Char) || 0) + 1)
        stringMap.set(s2Char, (stringMap.get(s2Char) || 0) + 1)
    }
    
    let left = 0;
    let right = s1.length - 1;
    
    while (right < s2.length) {
        if (isEqual(patternMap, stringMap)) {
            return true;
        }
        
        const firstCharInStr2 = s2.charAt(left);
        
        if (stringMap.get(firstCharInStr2) > 1) {
            stringMap.set(firstCharInStr2, stringMap.get(firstCharInStr2) - 1)
        } else {
            stringMap.delete(firstCharInStr2)
        }
        
        left++;
        right++;
        
        const lastCharOfStr2 = s2.charAt(right)
        stringMap.set(lastCharOfStr2, (stringMap.get(lastCharOfStr2) || 0) + 1)
    }
    
    return false;
};

function isEqual(map1, map2) {
    if (map1.size !== map2.size) {
        return false;
    }
    
    for (const key of map1.keys()) {
        if (!map2.get(key) || map1.get(key) !== map2.get(key)) {
            return false;
        }
    }
    
    return true;
}