underscore/countBy

(function(){
  const s1Array = [2, 3, 25, 12, 9];
  var s1 = _.countBy(s1Array, function(num){
  	if(num % 2 == 0){
  	  return "even"
  	} else {
  	  return "odd"
  	}
  });
  console.log(s1);
})();