Minimum Window Substring

/**
 * @param {string} s
 * @param {string} t
 * @return {string}
 */
var minWindow = function(s, t) {
    if (t.length > s.length) return '';
    
    const neededChars = {};
    
    for (let char of t) {
        neededChars[char] = (neededChars[char] || 0) + 1;
    }
    
    let left = 0;
    let right = 0;
    let neededLength = Object.keys(neededChars).length;
    let substring = '';
    
    while (right < s.length) {
        const rightChar = s[right];
        neededChars[rightChar]--;
        if (neededChars[rightChar] === 0) neededLength--;
        
        while (neededLength === 0) {
            if (!substring || substring.length > right - left + 1) {
                substring = s.slice(left, right + 1);
            }
            
            const leftChar = s[left];
            // If the leftChar in charMap is at exactly 0 before being 
            // incremented, we now need more leftChars so that its count
            // in charMap goes down to exactly 0
            if (neededChars[leftChar] === 0) {
                neededLength++;
            }
            neededChars[leftChar]++;
            left++;
               
        }
        
        right++;
    }
    
    return substring;
};