Cannot read property 'get' of undefined for unit testing in mocha

I am trying to write a unit test over my file routes.js using mocha and chai. I have defined a function in routes.js and has defined a test file in which my test case is there.

When i am running my test case, its showing the error TypeError: Cannot read property 'get' of undefined

My code for test.js is

var expect = require('chai').expect;
var chai = require('chai');
var app = ('../api/smartAccount/identifyLoan/getDirectDebitTrans/routes');

describe("Unit testing for function", function(){
    it("Testing the function using mocha", function(done){
        var req = {
            queryString: 101
        };

        var test = app.getUser.get(req);
        expect(test).to.be.an('undefined');
        done();
    }); 
});

I am passing req as i am expecting req.queryString.id in my routes.js .

code of routes.js is

var config = require("../../../../serverConfig/config");

module.exports = {
   getUser: {
        get: function(req, parameters, environment) {

        var id = req.queryString.id;

         //functionality
         });
  }
 }

Please help where i am going wrong.

p.s get function is fetching data from DB but i havent mentioned the bd connection as i feel its of irrelevance here. TIA