app.get is not a function in express.js - node.js

Question

I am actually not able to figure it out that why in index.js file in below code snippet is throwing me an error : app.get is not a function.
Please help me out..
//here is my app.js file
const express = require('express');
const app = express();
const helpers = require('./helpers');
const routes = require('./index')
app.use((req, res, next) => {
app.locals.h = helpers;
next();
});
app.use('/', routes);
app.set('views',(__dirname));
app.set('view engine', 'pug');
app.listen(3000,()=>console.log('port 3000'));
module.exports = app;
//here is my index.js file
const app = require('./app')
app.get('/',(req,res) => {
res.render('template');
})
module.exports = router;
//helpers.js
exports.title = "NODEjs";
//template.pug
doctype html
html
head
title=`${h.title}`
body
h1 myHeading #{h.title}

Answer 1

You have a circular dependency loop and rather than create an infinite loop, the require() subsystem detects that and fails to load your module.
In app.js, you load index.js. In index.js, you load app.js. Circular dependency loop.
There are two separate techniques that can be used to solve your particular problem. You appear to be using about some of one technique and some of another and that creates your problem.
A classic way to define new routes in a separate file is to just have that file create and export its own router. It then assigns the routes to the router (not to app) and thus that other file never needs the app object at all. Because you show module.exports = router, it appears you have part of that technique, but only part of it.
Here's how the code would work to do it that way:
// app.js
const express = require('express');
const app = express();
const helpers = require('./helpers');
app.use((req, res, next) => {
app.locals.h = helpers;
next();
});
// hook in routes from the index.js router
app.use('/', require('./index'));
app.set('views',(__dirname));
app.set('view engine', 'pug');
app.listen(3000,()=>console.log('port 3000'));
// index.js
const router = require('express').Router();
router.get('/',(req,res) => {
res.render('template');
});
module.exports = router;
You could also pass app to index.js when you load it rather than having it try to import app. This also solves the circular dependency issue.
const express = require('express');
const app = express();
const helpers = require('./helpers');
// pass app here so it can register routes
require('./index')(app);
app.use((req, res, next) => {
app.locals.h = helpers;
next();
});
app.use('/', routes);
app.set('views',(__dirname));
app.set('view engine', 'pug');
app.listen(3000,()=>console.log('port 3000'));
Change index.js to export a module constructor which you call and pass app to:
module.exports = function(app) {
app.get('/',(req,res) => {
res.render('template');
})
}

Answer 2

Add parenthesis on the first line :
const express = require('express')();

Answer 3

The first answer should be accepted.
This is happening only due to the circular dependencies in this case.