Working with Content Negotiation

app.get("/data", (req, res) => {
  switch (req.accepts(["json", "html", "xml"])) {
    case "json": return res.json(data);
    case "html": return res.render("data", { data });
    case "xml":  return res.type("xml").send(toXml(data));
    default:     return res.status(406).send("Not Acceptable");
  }
});

res.json({
  data: items,
  meta: { page, limit, total },
  links: { next: "/items?page=" + (page + 1) }
});

import { create } from "xmlbuilder2";
res.type("application/xml").send(
  create({ user: { id: user.id, name: user.name } }).end({ prettyPrint: true })
);

app.get("/users/:id", (req, res) => {
  const user = getUser(req.params.id);
  res.format({
    "application/json": () => res.json(user),
    "text/html":        () => res.render("user", { user }),
    "text/plain":       () => res.send(`${user.id}: ${user.name}`),
    default:            () => res.status(406).send("Not Acceptable")
  });
});

const accepted = req.accepts(["json", "html"]);
if (!accepted) return res.status(406).json({
  error: "Not Acceptable",
  supported: ["application/json", "text/html"]
});

function toCsv(rows) {
  if (!rows.length) return "";
  const headers = Object.keys(rows[0]);
  const escape = (v) => `"${String(v).replace(/"/g, '""')}"`;
  return [headers.join(","), ...rows.map(r => headers.map(h => escape(r[h])).join(","))].join("\n");
}

res.format({
  "application/json": () => res.json(rows),
  "text/csv": () => res.attachment("export.csv").type("csv").send(toCsv(rows))
});

app.use(express.json({ type: ["application/json", "application/*+json"] }));
app.use(express.urlencoded({ extended: true }));
app.use(express.text({ type: "text/csv" }));