This shrinks the window starting from the left to ensure we're only keeping the essential substring. We repeat this enlarging and then shrinking of the window, testing each one against our requirements.

This piece allows us to store the "best one" (by condition) found so far:

1if (end - begin + 1 < substrSize) { // overwrite if current substr is smaller
2  substrSize = end - begin + 1;
3  head = begin;
4}

At the end, we simply use the tracked data to get the substring we want:

1if (substrSize == s.length + 1) {
2    return "";
3} else {
4    return s.slice(head, head + substrSize);
5}