Here I have two js files first is app.js and second is secondfile.js.

app.js file is my main file in app.js file I wrote database connectivity code and other basic code. Now I include secondfile.js in app.js file and in secondfile.js I wrote one API but it doesn't work (it gives this type of errors ReferenceError: app is not defined) how to include secondfile.js in app.js?

app.js

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser'); 
var logger = require('morgan');
var indexRouter = require('./routes/index');
var usersRouter = require('./routes/users');
var mysql = require('mysql');
var bodyParser = require('body-parser');
var cors = require('cors');
var app = express();
const http = require('http');
const server = require('http').createServer(app);
var secondfile = require('./secondfile');

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'pug');
app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded( extended: false ));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));
app.use('/', indexRouter);
app.use('/users', usersRouter);
app.use(bodyParser.json());
app.use(bodyParser.urlencoded( extended: false ));
app.use(cors(Origin: 'http://127.0.0.1:3000'));

var connection = mysql.createConnection(
 host : 'localhost',
 user : 'root',
 password : '',
 database : 'rapnets',
);

connection.connect();

server.listen(3000,function(req,res)
 console.log('Connect on 3000');
);

module.exports = app;

secondfile.js

app.post('/getAllUsersDetails',function(req,res)
 connection.query('SELECT userid, username,inserteddate from user', function (error, results, fields) 
 if (error) throw error;
 res.send(
 'status':'1',
 'success': 'true',
 'payload': results,
 'message':'user details is selected'
 );
 );
);

In the secondFile export a function that adds the post API:

module.exports.setupApi = function(app) 
app.post('/getAllUsersDetails',function(req,res)
 connection.query('SELECT userid, username,inserteddate from user', function (error, results, fields) 
 if (error) throw error;
 res.send(
 'status':'1',
 'success': 'true',
 'payload': results,
 'message':'user details is selected'
 );
 );
);

In the app.js use second file as:

var secondfile = require('./secondfile');
........
secondfile.setupApi(app);