Minimum Window Substring

minWindow.js

/**
 * @param {string} s
 * @param {string} t
 * @return {string}
 */

var minWindow = function(s, t) {
  let map = new Map();
  let sLen = s.length;
  let tLen = t.length;
  let count = tLen;
  let min = Number.MAX_SAFE_INTEGER;
  let head = 0;
  let left = 0;
  let right = 0;

  if (!sLen || !tLen) {
      return '';
  }

  for (let i = 0; i < tLen; i++) {
    if (map.get(t[i]) === undefined) {
      map.set(t[i], 1)
    } else {
        map.set(t[i], map.get(t[i]) + 1);
    }
  }

  while (right < sLen) {
    if (map.get(s[right]) !== undefined) {
      if (map.get(s[right]) > 0) {
          count--;
      }
         map.set(s[right], map.get(s[right]) - 1);
    }

    right++;

    while (count === 0) {
      if (right - left < min) {
        min = right - left;
        head = left;
      }

      if (map.get(s[left]) !== undefined) {
        if (map.get(s[left]) === 0) {
            count++;
        }
           map.set(s[left], map.get(s[left]) + 1)
      }

      left++;
    }
  }

  return min === Number.MAX_SAFE_INTEGER ? '' : s.substr(head, min);
}