trackStream.js

export const streamChunk = function* (chunk, chunkSize) {
  let len = chunk.byteLength;

  if (!chunkSize || len < chunkSize) {
    yield chunk;
    return;
  }

  let pos = 0;
  let end;

  while (pos < len) {
    end = pos + chunkSize;
    yield chunk.slice(pos, end);
    pos = end;
  }
}

export const readBytes = async function* (iterable, chunkSize, encode) {
  for await (const chunk of iterable) {
    yield* streamChunk(ArrayBuffer.isView(chunk) ? chunk : (await encode(String(chunk))), chunkSize);
  }
}

export const trackStream = (stream, chunkSize, onProgress, onFinish, encode) => {
  const iterator = readBytes(stream, chunkSize, encode);

  let bytes = 0;

  return new ReadableStream({
    type: 'bytes',

    async pull(controller) {
      const {done, value} = await iterator.next();

      if (done) {
        controller.close();
        onFinish();
        return;
      }

      let len = value.byteLength;
      onProgress && onProgress(bytes += len);
      controller.enqueue(new Uint8Array(value));
    },
    cancel(reason) {
      onFinish(reason);
      return iterator.return();
    }
  }, {
    highWaterMark: 2
  })
}