Error

[hacker914]

Hi!! This is the error I am getting

  throw new TypeError('Router.use() requires a middleware function but got a ' + gettype(fn))
  ^

TypeError: Router.use() requires a middleware function but got a string
at Function.use (D:\Web Development\Complete Web Development Boot camp\Node.JS Backend\Lecture 72\node_modules\express\lib\router\index.js:458:13)
at Function. (D:\Web Development\Complete Web Development Boot camp\Node.JS Backend\Lecture 72\node_modules\express\lib\application.js:220:21)
at Array.forEach ()
at Function.use (D:\Web Development\Complete Web Development Boot camp\Node.JS Backend\Lecture 72\node_modules\express\lib\application.js:217:7)
at Object. (D:\Web Development\Complete Web Development Boot camp\Node.JS Backend\Lecture 72\app.js:16:5)
at Module._compile (internal/modules/cjs/loader.js:1072:14)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1101:10)
at Module.load (internal/modules/cjs/loader.js:937:32)
at Function.Module._load (internal/modules/cjs/loader.js:778:12)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:76:12)

My code is
//Using the previous code with few changes.

const express = require('express'); //Imported the module I installed in last leture
const path = require ('path');
const app = express(); //Created my app
const port = 80; //The port on which i want to listen.

//Change 1 : For serving static files
app.use('/static', express.static('static'));
// app.use("url", express.static("folder_name"));

// Setting template engine as pug
app.set('view engine', 'pug')

//Set the views directory
app.use('views', path.join(__dirname, 'views'));

//Our pug demo end point
app.get("/demo", (req, res)=>{
res.status(200).render('demo', { title: 'Hey Harry', message: 'Hello there and thanks for telling me how to use pubG!' })
});

app.get("/", (req, res)=>{
res.send("This is home page of express app")
})

app.get("/about", (req, res)=>{

res.status(200).send("This is get request of about page of express app")
})
app.post("/about", (req, res)=>{
res.send("This is post request of about page of express app")
})
app.get("/this", (req, res)=>{
res.status(404).send("Page not found");
})

app.listen(port,() =>{
console.log(My express app is successfully started at port ${port});
})

I have tried the
const router = express.router();
module.exports = router;

But still it is not working.