Content Negotiation with Accept Headers

APIs return JSON unconditionally. Clients that need XML, CSV, or specific JSON variants have no way to request their preferred format, requiring separate endpoints per format.

Implement content negotiation using the Accept request header to serve multiple formats from one endpoint.

app.get('/users', async (req, res) => {
  const users = await db.getUsers();

  // req.accepts() negotiates based on quality factors (q=0.9)
  const accept = req.accepts(['json', 'csv', 'xml']);

  switch (accept) {
    case 'json':
      res.json(users);
      break;
    case 'csv':
      res.setHeader('Content-Type', 'text/csv');
      res.setHeader('Content-Disposition', 'attachment; filename="users.csv"');
      res.send(toCsv(users));
      break;
    case 'xml':
      res.setHeader('Content-Type', 'application/xml');
      res.send(toXml(users));
      break;
    default:
      // 406 Not Acceptable
      res.status(406).json({
        error: 'Not Acceptable',
        supported: ['application/json', 'text/csv', 'application/xml'],
      });
  }
});

Content negotiation allows one endpoint to serve multiple formats. The client declares preference via Accept header with quality factors; the server responds with the best match or 406.