How would you implement resumable/chunked file uploads in Node.js to handle very large files over unreliable network connections?

Chunked (resumable) uploads split a large file into smaller pieces on the client side, uploading each chunk as a separate request with metadata identifying its position within the overall file -- the server stores each received chunk (or appends it directly to a partially-assembled file) and, once all chunks have arrived, reassembles them into the complete file; if the connection drops mid-upload, only the remaining unsent chunks need to be retried, rather than restarting the entire upload from scratch.

app.post('/upload-chunk', upload.single('chunk'), async (req, res) => {
  const { uploadId, chunkIndex, totalChunks } = req.body;
  await fs.promises.rename(req.file.path, `./chunks/${uploadId}-${chunkIndex}`);

  const receivedChunks = await fs.promises.readdir('./chunks');
  if (receivedChunks.filter(f => f.startsWith(uploadId)).length === Number(totalChunks)) {
    await assembleChunksIntoFinalFile(uploadId, totalChunks);
  }
  res.json({ received: chunkIndex });
});