throw new MongooseError('The `uri` parameter to `openUri()` must be a ' + - node.js

Question

i can't find the solution.please help..
MongooseError: The uri parameter to openUri() must be a string, got "undefined". Make sure the first parameter to mongoose.connect() or mongoose.createConnection() is a string.
if (process.env.NODE_ENV !== 'production') {
require('dotenv').load()
}
const express = require('express')
const app = express()
const expressLayouts = require('express-ejs-layouts')
const bodyParser = require('body-parser')
const methodOverride = require('method-override')
const indexRouter = require('./routes/index')
const authorRouter = require('./routes/authors')
const bookRouter = require('./routes/books')
app.set('view engine', 'ejs')
app.set('views', __dirname + '/views')
app.set('layout', 'layouts/layout')
app.use(expressLayouts)
app.use(methodOverride('_method'))
app.use(express.static('public'))
app.use(bodyParser.urlencoded({ limit: '10mb', extended: false }))
const mongoose = require('mongoose')
mongoose.connect(process.env.DATABASE_URL, { useNewUrlParser: true })
const db = mongoose.connection
db.on('error', error => console.error(error))
db.once('open', () => console.log('Connected to Mongoose'))
app.use('/', indexRouter)
app.use('/authors', authorRouter)
app.use('/books', bookRouter)
app.listen(process.env.PORT || 3000)

Answer 1

Create the .env file on your root project. Then under that file copy-paste this line i.e
DATABASE_URL="192.168.1.1:3000"
then on main server file may be app.js or server.js or somethingelse.js file before the line i.e const mongoose = require('mongoose') copy-paste this line
require('dotenv').config();
and then console.log(process.env.DATABASE_URL)// This will return your Database URL
Hope you understand the concept.