Unit testing express.js route with sinon

Question

I am learning express and sinon (the hard way :) ). I'd like to unit test the following code:

//  ./routers/root.js

var express = require('express');
var router = express.Router();

router
  .route('/')
  .get(function(req, res) {
    res.writeHead(200, {'Content-Type': 'application/json'});
    res.write(JSON.stringify({Application: "NodeTours", Version: "2.0.0", Host: host, Port: port}));
    res.end();
  });

module.exports = router;

I've been trying different things without success. I'm back to the basic example. I realize it may seem like a complete newbie question but I'm clueless. Here's the latest try:

//   ./test/test.js

var sinon = require('sinon')
var expect = require('chai').expect

describe('Routers', function() {

  context('/', function() {
    var rootRouter = require('../routers/root')
    var res = {Application: "NodeTours", Version: "2.0.0", Host: 'host', Port: 'port'};
    it('should call / router', function() {
      rootRouter.route('/').get(null, res);
      // TODO: expect...
    })
  })
})

It fails with:

  Routers
    /
      1) should call / router


  0 passing (4ms)
  1 failing

  1) Routers
       /
         should call / router:
     Error: Route.get() requires a callback function but got a [object Null]
      at Route.<computed> [as get] (node_modules/express/lib/router/route.js:202:15)
      at Context.<anonymous> (test/test.js:10:29)
      at processImmediate (internal/timers.js:456:21)

I've tried multiple tutorials without success. I'd appreciate any pointers to how my code could be unit tested. Or how to adjust my code so unit testing is easier/possible.

My main app server.js file for context and reference (removed code not relevant for this question)

//   ./server.js
// npm pacakages
var express = require('express');
var bodyParser = require('body-parser');

// app imports
var { rootRouter } = require('./routers');
// other cut out

//globals
app = express();
app.use(bodyParser.json());
app.use('/', rootRouter);
// other cut out

// Set up Mongo DB connection
// code to st up db connection

// Start server (only if connection to DB established)
var server = MongoClient(url, { useUnifiedTopology: true, poolSize: 10 }).connect().then(client => {
  const dbo = client.db(dbName);

  // some init code cut out

  // make connection available
  app.locals.dbo = dbo;

  // start server
  app = app.listen(process.env.PORT || 7777, function () {
    host = require('os').hostname();
    port = "7777";
    logger.verbose("Startup: NodeTours listening at http://%s:%s", host, port)
  });
}).catch(error => {
  logger.error("Startup: Couldn't connect to the DB. The app will exit")
  logger.error("  Error: " + error)
  process.exit();
  }
);

Answer 1

In your test case, you are attempting to call your GET handler directly:

it('should call / router', function() {
  rootRouter.route('/').get(null, res);
  // TODO: expect...
})

But appRouter.route('/').get() is a method intended to install a request handler, not to call it. That's why the error you are getting says:

Error: Route.get() requires a callback function but got a [object Null]

i.e., you are passing null for what you think is the request parameter, but which Express expects will be a method (callback) handler.

Normally what I will do is define my handlers as non-anonymous functions:

function rootHandler(req, res) {
    res.writeHead(200, {'Content-Type': 'application/json'});
    res.write(JSON.stringify({Application: "NodeTours", Version: "2.0.0", Host: host, Port: port}));
    res.end();
}

router
  .route('/')
  .get(rootHandler);

...and then unit tests can simply invoke the function directly, rather than go through Express (because normally we're interested in testing our own code, not the framework):

it('should do something with root handler', function() {
  rootHandler(null, res);
  // TODO: expect...
})

If you really want to test the routing logic, then using supertest as suggested in the comments is a good option.