Express 4 - chaining res.json with promise.then does not work

I'm working on an express 4 app that uses mysql and sequelize packages. Sequelize ORM uses promises to fetch data from database. I'm trying to fetch data in router and send json response. When I try to chain then callback of promise with res.json I get an error in console saying Unhandled rejection TypeError: Cannot read property 'get' of undefined

// This works
employeeRouter.get("/:id", function(req, res){
   Employee.findById(req.params.id).then(function(data){
      res.json(data);
   });
});

// Replacing above code with following doesn't work
employeeRouter.get("/:id", function(req, res){
   Employee.findById(req.params.id).then(res.json);
});

Error Stack:

Unhandled rejection TypeError: Cannot read property 'get' of undefined
    at json (D:\Workstation\DataPro\CountryStats\node_modules\express\lib\response.js:241:21)
    at tryCatcher (D:\Workstation\DataPro\CountryStats\node_modules\bluebird\js\release\util.js:16:23)
    at Promise._settlePromiseFromHandler (D:\Workstation\DataPro\CountryStats\node_modules\bluebird\js\release\promise.js:504:31)
    at Promise._settlePromise (D:\Workstation\DataPro\CountryStats\node_modules\bluebird\js\release\promise.js:561:18)
    at Promise._settlePromise0 (D:\Workstation\DataPro\CountryStats\node_modules\bluebird\js\release\promise.js:606:10)
    at Promise._settlePromises (D:\Workstation\DataPro\CountryStats\node_modules\bluebird\js\release\promise.js:685:18)
    at Async._drainQueue (D:\Workstation\DataPro\CountryStats\node_modules\bluebird\js\release\async.js:138:16)
    at Async._drainQueues (D:\Workstation\DataPro\CountryStats\node_modules\bluebird\js\release\async.js:148:10)
    at Immediate.Async.drainQueues [as _onImmediate] (D:\Workstation\DataPro\CountryStats\node_modules\bluebird\js\release\async.js:17:14)
    at processImmediate [as _immediateCallback] (timers.js:383:17)

models/employee.js

var Sequelize = require('sequelize'),
    sequelize = require('../db-connect/sequelize');

(function(){

  // Use Strict Linting
  'use strict';

  // Define Sequalize
  var Employee = sequelize.define('employee', {
    empNo: { field: 'emp_no', type: Sequelize.INTEGER, primaryKey: true, autoIncrement: true },
    birthDate: { field: 'birth_date', type: Sequelize.DATE },
    firstName: { field: 'first_name', type: Sequelize.STRING },
    lastName: { field: 'last_name', type: Sequelize.STRING },
    gender: { field: 'gender', type: Sequelize.ENUM('M', 'F') },
    hireDate: { field: 'hire_date', type: Sequelize.DATE },
  });

  // Export
  module.exports = Employee;

}());

db-connect/sequelize.js

var Sequelize = require('sequelize');

(function(){

  // Use Strict Linting
  'use strict';

  // Sequalize Connection
  var sequelize = null;

  // Create Sequalize Connection
  if(!sequelize){
    sequelize = new Sequelize('employees', 'root', '', {
      host: 'localhost',
      dialect: 'mysql',
      define: {
        timestamps: false
      }
    });
  }

  module.exports = sequelize;

}());

routes/employees.js

var express = require('express'),
    Employee = require('../models/employee');

(function(app){

  // Use Strict Linting
  'use strict';

  // Create Router
  var employeeRouter = express.Router();

  // Home Page
  employeeRouter.get("/", function(req, res){
    res.json({employees: ['all']});
  });

  // Get Specific Employee
  employeeRouter.get("/:id", function(req, res, next){
    Employee.findById(req.params.id).then(function(data){
      res.json(data);
    });
  });

  // ----------------------------------
  // Export
  // ----------------------------------

  module.exports = employeeRouter;

}());

When you pass res.json as a function, the res object gets lost and thus when json() executes, it has no object and you get the error you are seeing. You can fix that by using .bind():

employeeRouter.get("/:id", function(req, res){
   Employee.findById(req.params.id).then(res.json.bind(res));
});

This will make sure that the res object stays with your method when the method is executed. Using .bind() as above is essentially the same as:

employeeRouter.get("/:id", function(req, res){
   Employee.findById(req.params.id).then(function(data) {
       return res.json(data);
   });
});

In fact, .bind() actually creates a stub function like the anonymous one in the above example. It just does it for you rather than making you do it.

---

To further example, let's say you had two separate res objects, res1 and res2 from two separate requests.

var x = res1.json;
var y = res2.json;

console.log(x === y);    // true, no association with either res1 or res2 any more

This is because referencing res1.json just gets a reference to the .json method. It uses res1 to get that method (which is fetched from the res1 prototype, but once it has the method, it's just a pointer to the method and there is no longer an association with the object that contained the method. So, when you pass res.json to a function, you get no attachment to res. Then when the function you passed res.json to goes to actually call your function it calls it like this:

var z = res.json;
z();

And, when z() is called, the this value inside of json ends up being undefined and there is no connection to the res object. Using .bind() creates a stub function that calls it as res.json(...) to keep the connection to the object and make sure this is set properly when the method is executed.