Testing express.Router() with chai gives "TypeError: res.send is not a function"?

I have a controller/route file like this..

const express = require('express');
const pService = require('../../services/parameter/service');
let router = express.Router()
router.get('/valuebyobject/:main/:sub?', pService.getValue); 

and the getValue function is like this below..

const getValue = async (req, res, next) => {
    try {
         let ts = Date.now();
         const obj= {};
         obj.utime = ts;
         obj.data =[];
         const valueMain = req.params.main;
         const sub = req.params.sub;
         var jsonPath = path.join(__dirname, '..','..', 'data','Format_'+valueMain+'.json');
         fs.readFile(jsonPath, (err, data) => {
            if (err) return res.status(404).json({
                'code': 'NOT_FOUND',
                'description': `Required Main Parameter: ${valueMain} result is empty`
            });
            let file = JSON.parse(data);
            if(sub){  //If 'sub' param is not empty, take both 'main' and 'sub'
                    const filterData = file['Data'].filter(element=>{
                    if (element.Id == sub)  //Filtering according to Main & Sub params
                           obj.data.push({'My': parseInt(valueMain), 'Other': parseInt(element.Id), 'Value': element.Value})
                           return element;
                    })
                    if(typeof filterData != "undefined" && filterData != null && filterData.length != null && filterData.length > 0)  //Checking if array is not empty
                      {
                           console.log(obj)
                           res.send(obj);
                    }
                    else {  // If array is empty send 404 back
                           return res.status(404).json({
                                  'code': 'NOT_FOUND',
                                  'description': `Required Sub Parameter ${sub} result is empty`
                            });
                    }
            }
            else {  // if 'sub' parameter is empty then query with only 'main' param
                    const filterData = file['Data'].filter(element=>{
                    obj.data.push({'My': parseInt(valueMain), 'Other': parseInt(element.Id), 'Value': element.Value})
                    return element;
                    })
                    if(typeof filterData != "undefined" && filterData != null && filterData.length != null && filterData.length > 0)  //Checking if array is not empty
                    {
                            console.log(obj)
                            res.send(obj);
                    }
                    else {  // If array is empty send 404 back
                        return res.status(404).json({
                            'code': 'NOT_FOUND',
                            'description': `Required Sub Parameter ${sub} result is empty`
                        });
                    }
            }
        }); 
    } catch (err) {
        return res.status(500).json({
            'code': 'SERVER_ERROR',
            'description': 'something went wrong, Please try again'
        });
    }
}

I am testing the following route with chai and I have a test file like this below..

const chai = require("chai");
const chaiHttp = require("chai-http");
const expect = chai.expect;
const server = require("../controllers/apis/parameter");

chai.should();
chai.use(chaiHttp);

let main= 101
describe('Parameter APIs', () => {

    describe('Test GET route (/valuebyobject', () => {

        it('should return all ibjects', async () => {
            let res = await chai
                .request(server)
                .get(`/valuebyobject/${main}`)
                expect(res.status).to.equal(200)
        
        
        });
    });

});

By running the test file it gives me this error:

1. Parameter APIs Test GET route (/valuebyobject should return all objects: Uncaught TypeError: res.send is not a function

But I am not able to figure out the solution how I can test my API with chai and to avoid this TypeError.

But changinging the express.Router() to normal express() in controller file from above make it work.

const express = require('express');
const pService = require('../../services/parameter/service');
let router = express()
router.use(express.json())
router.get('/valuebyobject/:main/:sub?', pService.getValue);

But i would like to use expres.Router() in my application. Can somebody please help how to test endpoints of express.Router()?