`mongoose.connect()` fires error: first argument should be String, received - node.js

Question

throw new MongooseError('The uri parameter to openUri() must be
a ' +
^
MongooseError: The uri parameter to openUri() must be a string,
got "undefined". Make sur
first parameter to mongoose.connect() or mongoose.createConnection() is a string.
at NativeConnection.Connection.openUri (C:\Users\niko\Desktop\opaa\node_modules\mongoose\nnection.js:694:11)
at _mongoose._promiseOrCallback.cb (C:\Users\niko\Desktop\opaa\node_modules\mongoose\lib\js:351:10)
at promiseOrCallback (C:\Users\niko\Desktop\opaa\node_modules\mongoose\lib\helpers\promislback.js:10:12)
at Mongoose._promiseOrCallback (C:\Users\niko\Desktop\opaa\node_modules\mongoose\lib\inde at
Mongoose.connect
(C:\Users\niko\Desktop\opaa\node_modules\mongoose\lib\index.js:350:20)
at Object. (C:\Users\niko\Desktop\opaa\app.js:9:10)
at Module._compile (internal/modules/cjs/loader.js:778:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
at Module.load (internal/modules/cjs/loader.js:653:32)
at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
at Function.Module._load (internal/modules/cjs/loader.js:585:3)
at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
at startup (internal/bootstrap/node.js:283:19)
at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)
const { response } = require('express');
const express = require('express');
const path = require('path');
const dotenv = require('dotenv')
const mongoose = require('mongoose');
dotenv.config();
mongoose.connect( process.env.DB_CONNECT, { useNewUrlParser: true }, () =>{
console.log("connected to db");
});
const route = require('./routers/routers');
const { connect } = require('./routers/routers');
const app = express();
app.use(express.urlencoded({ extended: false }));
app.use(express.json());
app.set("views", "views")
app.set("view engine", "hbs")
app.use(express.static('public'));
app.use('/user', route);
app.listen(3000, () => console.log("Server Is Runing on localhost:3000"));

Answer 1

You may try to be sure you've passed the DB_CONNECT.
you may try DB_CONNECT=YOUR_DB_URI node index.js
also, you may check with console.log(process.env.DB_CONNECT) at the beginning of your script.

Answer 2

The error message tells that the mongoose.connect received URL is undefined. I suggest you instead process.env.DB_CONNECT try passing an URL to the DB for testing purpose see if that works.
The issue is with the process.env.DB_CONNECT is not setting up correctly, you might need to check dotenv configuration.