Stubbing module functions in a node server for unit testing edge cases

lib.js

var lib = function() {};

lib.prototype.sayHi = function() {
  return 'Hello from the original module';
};

module.exports = lib;

package.json

{
  "dependencies": {
    "express": "*",
    "sinon": "*",
    "supertest-as-promised": "*",
    "should": "*",
  }
}

server.js

var Lib = require('./lib');
var app = require('express')();
var server = {};
var runningInstance;

app.get('/hello', function(req, res) {
  var lib = new Lib();
  res.json(lib.sayHi());
});

server.start = function(cb) {
  runningInstance = app.listen(7777, cb);
};

server.close = function(cb) {
  if(!runningInstance) return cb(null);
  runningInstance.close(cb);
};

module.exports = server;

test.js

require('should');
var request = require('supertest-as-promised');
var sinon = require('sinon');
var server = require('./server');
var Lib = require('./lib') ;

before(function(done) {
  server.start(done);
});

after(function(done) {
  server.close(done);
});

it('Replies on GET /hello', function(done) {
  request('localhost:7777')
    .get('/hello')
    .expect(200)
    .then(function(res) {
      res.body.should.containEql('Hello from the original module');
      done();
    }).catch(done);
});

it('Can be mocked', function(done) {
  sinon.stub(Lib.prototype, 'sayHi', function() {
    return 'Hello from the stub!';
  });
  request('localhost:7777')
    .get('/hello')
    .expect(200)
    .then(function(res) {
      res.body.should.containEql('Hello from the stub!');
      Lib.prototype.sayHi.restore();
      done();
    }).catch(done);
});