code: function () {
      // Combine the buffers we've received and reset the list of buffers.
      var combinedBuffer = Buffer.concat(receivedBuffers, receivedBuffersLength);
      receivedBuffers.length = 0; // Trick to reset the array while keeping the original reference
      receivedBuffersLength = 0;
  
      if (combinedBuffer.length > partSizeThreshold) {
        // The combined buffer is too big, so slice off the end and put it back in the array.
        var remainder = new Buffer(combinedBuffer.length - partSizeThreshold);
        combinedBuffer.copy(remainder, 0, partSizeThreshold);
        receivedBuffers.push(remainder);
        receivedBuffersLength = remainder.length;
  
        // Return the perfectly sized part.
        var uploadBuffer = new Buffer(partSizeThreshold);
        combinedBuffer.copy(uploadBuffer, 0, 0, partSizeThreshold);
        return uploadBuffer;
      }
      else {
        // It just happened to be perfectly sized, so return it.
        return combinedBuffer;
      }
    }
docs: Take a list of received buffers and return a combined buffer that is exactly partSizeThreshold in size.