res.download for file downloads in Express - Performance & Optimization

Performance: res.download for file downloads

MEDIUM IMPACT

This affects how quickly a file starts downloading and how server resources are used during the download process.

Serving a file for user download in an Express app

Express

app.get('/file', (req, res) => {
  res.download('/path/to/file.pdf', 'file.pdf', err => {
    if (err) res.status(500).send('Error downloading file');
  });
});

Streams the file directly to the response, freeing memory and allowing faster download start.

📈 Performance GainNon-blocking, low memory use, faster response start

Serving a file for user download in an Express app

Express

app.get('/file', (req, res) => {
  const file = fs.readFileSync('/path/to/file.pdf');
  res.setHeader('Content-Disposition', 'attachment; filename="file.pdf"');
  res.send(file);
});

Reads the entire file into memory before sending, blocking the event loop and increasing memory use.

📉 Performance CostBlocks server during read, high memory usage for large files, slower response start

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
res.send with fs.readFileSync	0 (server-side only)	0	0	[X] Bad
res.download streaming file	0 (server-side only)	0	0	[OK] Good

Rendering Pipeline

The server streams the file data directly to the client without buffering the entire file, minimizing server blocking and memory pressure.

→Server File I/O

→Network Transfer

⚠️ BottleneckServer File I/O if file is large or on slow disk

Optimization Tips

1Stream files with res.download to reduce server memory use and speed up download start.

2Avoid reading entire files into memory before sending to prevent blocking the server.

3File downloads do not impact Core Web Vitals related to page rendering.

Performance Quiz - 3 Questions

Why is using res.download better than reading the file fully into memory before sending?

A Because res.download streams the file, reducing memory use and speeding up response start

B Because res.download compresses the file automatically

C Because res.download caches the file in the browser

D Because res.download encrypts the file during transfer

DevTools: Network

How to check: Open DevTools, go to Network tab, request the file download endpoint, and observe the timing and size of the response.

What to look for: Look for fast 'Time to First Byte' and steady data transfer indicating streaming rather than delayed full buffering.