A utility function that creates/reads a byte stream (eg request.body) with larger chunks (defaults to 256KB)

worker.ts

export default {
  async fetch(request: Request) {
    const chunkedRequestBody = request.body && chunkStream(request.body);
    return new Response(chunkedRequestBody);
  },
};

// By default will emit chunks of length `chunkSize` (default 256KB).
// If you wish to emit as soon as available, you can pass in `allowPartial: true`,
// you may just end up with more buffers, some smaller than `chunkSize`.

function chunkStream(
  sourceStream: ReadableStream<Uint8Array>,
  { chunkSize = 256 * 1024, allowPartial = false }: { chunkSize?: number; allowPartial?: boolean } = {},
): ReadableStream<Uint8Array> {
  const reader = sourceStream.getReader({ mode: "byob" });
  return new ReadableStream({
    async pull(controller) {
      const { done, value } = await reader.read(
        new Uint8Array(new ArrayBuffer(chunkSize)),
        // @ts-expect-error 2nd arg currently missing from @cloudflare/workers-types
        allowPartial ? undefined : { min: chunkSize },
      );
      if (value != null && value.byteLength > 0) {
        controller.enqueue(new Uint8Array(value.buffer, 0, value.byteLength));
      }
      if (done) {
        controller.close();
      }
    },
    cancel(reason) {
      return reader.cancel(reason);
    },
  });
}

// Or a class version if you want to do `new ChunkedStream(sourceStream)`

class ChunkedStream extends ReadableStream<Uint8Array> {
  constructor(
    sourceStream: ReadableStream<Uint8Array>,
    { chunkSize = 256 * 1024, allowPartial = false }: { chunkSize?: number; allowPartial?: boolean } = {},
  ) {
    const reader = sourceStream.getReader({ mode: "byob" });
    super({
      async pull(controller) {
        // as above...
      },
      cancel(reason) {
        // as above...
      },
    });
  }
}