why does express middleware not working in separate files - node.js

Question

I want to broke down my routers into separate file rather than keeping all API routes in a single file. So i tried to identify user URL using a middle ware and call the api according to the url as you seeing bellow but middleware function is not working . how to solve this?
//HERE IS INDEX.jS file CODE
const express = require("express");
const dotenv = require("dotenv");
const app = express();
const PORT = process.env.PORT || 7000;
app.listen(PORT);
app.use("/users", require("./routes/users/users"));
//==========================================================
//HERE IS users.js FILE CODE
const express = require("express");`enter code here`
const router = require("express").Router();
express().use(selectApi);
function selectApi(req, res, next) {
console.log("this line also not executing")
switch(req.originalUrl){
case '/':
// calling api here from a nother separate file
case '/user/:id'
// calling api here from a nother separate file
}
}
module.exports = router;

Answer 1

There is an example in the Express.js Routing Guide. I've modified it slightly to fit your example.
users.js
var express = require('express')
var router = express.Router()
// middleware that is specific to this router
router.use(function timeLog (req, res, next) {
console.log('Time: ', Date.now())
next()
})
// define the home page route
router.get('/', function (req, res) {
// calling api here from a nother separate file
})
router.get('/user/:id', function (req, res) {
// calling api here from a nother separate file
})
module.exports = router
index.js
const express = require("express");
const dotenv = require("dotenv");
const app = express();
const PORT = process.env.PORT || 7000;
app.listen(PORT);
app.use("/users", require("./routes/users/users"));