node-server.js

// server.js
const jsonServer = require("json-server");
const server = jsonServer.create();
const router = jsonServer.router("db.json");
const middlewares = jsonServer.defaults();

server.use(middlewares);

server.listen(3001, () => {
  console.log("JSON Server is running");
});

server.put("/customers/30000001", (req, res) => {
  let body = [];
  req
    .on("data", chunk => {
      body.push(chunk);
    })
    .on("end", () => {
      body = JSON.parse(Buffer.concat(body).toString());
      console.log(JSON.stringify(body));
      if (body.age && body.age > 18) {
        console.log("error de validación");
        return res.send({
          error: true,
          validation: {
            age: "Debe ser menor de edad",
            name: "El nombre es incorrecto"
          }
        });
      } else {
        res.send("ok");
      }
    });
});

server.use(router);