The general chunker — Elixir's chunk_while/4, object-shaped for JS: step returns { acc } to keep accumulating, { acc, emit } to emit a chunk, plus halt: true to end the stream; flush may emit one last chunk from the final accumulator. Values only; failures pass through.

const batchesOfTwo = Stream.from([1, 2, 3, 4, 5]).chunkWhile(
  [] as number[],
  (n, acc) => (acc.length === 1 ? { acc: [], emit: [...acc, n] } : { acc: [...acc, n] }),
  (acc) => (acc.length > 0 ? acc : undefined),
);
await batchesOfTwo.collect(); // [[1, 2], [3, 4], [5]]