What is content negotiation in HTTP, and how would a Node.js API implement it to return either JSON or XML based on a client's Accept header?

Content negotiation lets a single endpoint serve different representations of the same underlying resource based on what the client indicates it prefers via the Accept request header -- an Express route can inspect req.accepts() (which parses and matches against the Accept header) to determine the client's preferred format and respond accordingly, returning a 406 Not Acceptable if none of the server's supported formats match what the client will accept.

app.get('/users/:id', (req, res) => {
  const user = getUserData(req.params.id);
  res.format({
    'application/json': () => res.json(user),
    'application/xml': () => res.send(convertToXml(user)),
    default: () => res.status(406).send('Not Acceptable'),
  });
});