Check the equality for each subset of a string S against the target T and return a count
I'd just like to point out, that the code IMHO has far too many "length optimizations". Just take the first line:

if (!s.length) return +(!t.length);


I've been a JS developer for over 20 years and I couldn't say off the top of my head what + does to a boolean. It would be much more readable as:

if (s.length === 0) {
  return (t.length === 0) ? 1 : 0;
}


Also a comment why these are the correct return values would be very helpful.