Get partial range of ReadableStream

I want to return a specific byte range given a ReadableStream. It sounds easy but I can't find any way to skip through or read a specific amount of bytes from the stream since we can only read chunks. I would prefer to not store any data and just send the stream. It sounds like I could do this with a TransformStream and PipeThrough(). But I need some help wrapping my head around it.

Example: Given a ReadableStream of say 1000 bytes, I want to return another stream that starts at byte 300 and ends at byte 900.

I know this can easily be done with nodes createReadableStream(), but I need to run this in the browser so it cant use node.

Here's some skeleton code to get you started:

function byteRangeTransform(start, end) {
  let bytesSeen = 0;
  return new TransformStream({
    transform(chunk, controller) {
      const chunkStart = bytesSeen;
      const chunkEnd = bytesSeen + chunk.byteLength;
      bytesSeen += chunk.byteLength;
      
      // Six cases:

      // 1. Chunk entirely before start
      if (chunkEnd < start) {
        return;
      }
      
      // 2. Chunk starts before start, ends between start and end
      if (chunkStart < start && chunkEnd >= start && chunkEnd <= end) {
        const slice = /* TODO */;
        controller.enqueue(slice);
        return;
      }
      
      // 3. Chunk starts before start, ends after end
      if (chunkStart < start && chunkEnd > end) {
        // TODO
      }

      // 4. Chunk starts after start, ends between start and end
      // 5. Chunk starts after start, ends after end
      // 6. Chunk starts after end
    }
  });
}

const onlyInRange = originalReadable.pipeThrough(byteRangeTransform(300, 900));