minWindow = function (s, t) {
    const isMissingArgs = !s.length || !t.length;
    if (isMissingArgs) return '';

    const frequencyMap = getFrequencyMap(t);
    const { start, end } = getWindowPointers(s, t, frequencyMap);

    return getSubString(s, start, end);
}
https://leetcode.com/problems/minimum-window-substring/
Time O(N + M) | SpaceO(N + M)
@param {string} s
@param {string} t
@return {string}