server.js

var jsonServer = require('json-server')
var server = jsonServer.create()
var router = jsonServer.router('db.json')
var middlewares = jsonServer.defaults()

function simpleAuth(req, res, next) {

  if (req.headers.authorization) {

    // user and password are stored in the authorization header
    // do whatever you want with them
    var user_and_password = new Buffer(req.headers.authorization.split(" ")[1], 'base64').toString();

    // for example, get the username
    var user_only = user_and_password.split(':')[0];


    /*
     *  I am not sure if you want to only send the user as a simple confirmation
     *  or you want to apply your own logic, like, really authenticate/validate
     *  the user against users database, static users .. etc
     *
     *  in production, it is recommended to validate the user by somehow.
     */


    // and save it in the request for later use in the `router.render` method
    req.user = user_only;

    // continue doing json-server magic
    next();

  } else {
    // it is not recommended in REST APIs to throw errors,
    // instead, we send 401 response with whatever erros
    // we want to expose to the client
    res.status(401).send({error: 'unauthorized'})
  }
}

// this method overwrites the original json-server way of response
// with your custom logic, here we will add the user to the response
router.render = function (req, res) {

  // manually wrap any response send by json-server into an object
  // this is something like `res.send()`, but it is cleaner and meaningful
  // as we're sending json object not normal text/response
  res.json({
    user: req.user, // the user we stored previously in `simpleAuth` function
    body: res.locals.data // the original server-json response is stored in `req.locals.data`
  })
}

// start setting up json-server middlewares
server.use(middlewares)

// before proceeding with any request, run `simpleAuth` function
// which should check for basic authentication header .. etc
server.use(simpleAuth);

// continue doing json-server magic
server.use(router);

// start listening to port 3000
server.listen(3000, function () {
  console.log('JSON Server is running on port 3000');
})