Why Function.prototype.bind is useful (but mostly how I set up my node.js projects)

accounts.js

module.exports = {
    // lots of other files are also exported
    localLogin: function (passport, req, res, next) {
        passport.authenticate("login", {
            successRedirect: "/order/admin",
            failureRedirect : "/order/admin/login",
            failureFlash : true
        })(req, res, next);
    },
    localSignup: function (passport, req, res, next) {
        passport.authenticate("register", {
            successRedirect : "/order/admin",
            failureRedirect : "/order/register",
            failureFlash : true
        })(req, res, next);
    }
}

routes.js

var r = require("routing.js")

module.exports = function (app, passport) {
  // lots of routes here, e.g.:
  app.get(["/", "/order"], r.general.index);
  
  // to register (as an admin)
  app.route("/order/register")
      .get(r.accounts.getRegister)
      // note the use of `Function.prototype.bind`
      .post(r.accounts.localSignup.bind(null, passport));

  // to login (as an admin)
  app.route("/order/admin/login")
      .get(r.accounts.getLogin)
      .post(r.accounts.localLogin.bind(null, passport));
  
  
}

routing.js

// main routing file
var rekwire = require("rekwire");

module.exports = {
      accounts   : rekwire("app/routes/accounts")
    , admin      : rekwire("app/routes/admin")
    , api        : rekwire("app/routes/api")
    , errors     : rekwire("app/routes/errors")
    , general    : rekwire("app/routes/general")
    , middleware : rekwire("app/routes/middleware")
    , params     : rekwire("app/routes/params")
}

server.js

// require passport, since it's being use to authenticate
var passport = require("passport")

// start express
var app = module.exports = express();

// config/passport.js configures the library
require("./config/passport")(passport);

// initializes passport and starts a new session
app.use(passport.initialize());
app.use(passport.session());

// require my routing file, routes.js
require("./app/routes.js")(app, passport);

/* lots of other code is midding */