var numbers = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21];
Rx.Observable
  .from(numbers)
  .bufferCount(5)
  .reduce(function(stats, buffered) {
    var evenCount = stats.evens;
    var oddCount = stats.odds;
    for (var i = 0; i < buffered.length; i += 1) {
      if (buffered[i] % 2 === 0) {
        evenCount += 1;
      } else {
        oddCount += 1;
      }
    }
    return { evens: evenCount, odds: oddCount };
  }, { evens: 0, odds: 0 }).subscribe(
    function(value) { console.log(value); },
    function(error) { console.error(error.toString() + error.stack); }
)