How do I fix this error? Mongooseconnection error - node.js

Question

I am trying to run my application and it keeps giving me this error and this is from a demo app from sitepoint. This is the details of the app
https://www.sitepoint.com/build-simple-beginner-app-node-bootstrap-mongodb/
The error:
throw new MongooseError('The `uri` parameter to `openUri()` must be a ' +
^
MongooseError: The `uri` parameter to `openUri()` must be a string, got "undefined". Make sure the first parameter to `mo
ngoose.connect()` or `mongoose.createConnection()` is a string.
at new MongooseError (C:\Users\tychi\WebstormProjects\demo-node-app\node_modules\mongoose\lib\error\mongooseError.js:
10:11)
at NativeConnection.Connection.openUri (C:\Users\tychi\WebstormProjects\demo-node-app\node_modules\mongoose\lib\conne
ction.js:579:11)
at Mongoose.connect (C:\Users\tychi\WebstormProjects\demo-node-app\node_modules\mongoose\lib\index.js:333:15)
at Object.<anonymous> (C:\Users\tychi\WebstormProjects\demo-node-app\start.js:4:10)
at Module._compile (internal/modules/cjs/loader.js:688:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:699:10)
at Module.load (internal/modules/cjs/loader.js:598:32)
at tryModuleLoad (internal/modules/cjs/loader.js:537:12)
at Function.Module._load (internal/modules/cjs/loader.js:529:3)
at Function.Module.runMain (internal/modules/cjs/loader.js:741:12)
at startup (internal/bootstrap/node.js:285:19)
at bootstrapNodeJSCore (internal/bootstrap/node.js:739:3)
[nodemon] app crashed - waiting for file changes before starting...
the code:
require('dotenv').config();
const mongoose = require('mongoose');
mongoose.connect(process.env.demoapp ,
{ useNewUrlParser: true },
{ useUnifiedTopology: true })
mongoose.connection
.on('open', () => {
console.log('Mongoose connection open');
})
.on('error', (err) => {
console.log(`Connection error: ${err.message}`);
});
require('./models/Registration');
const app = require('./app');
const server = app.listen(3000, () => {
console.log(`Express is running on port ${server.address().port}`);
});