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.
mongoose.connect(), first argument should be String, received undefined
//If .env file is used in config folder, make sure the DB_STRING is associated with its
//value, which is the mongodb connection string from the cluster
//Next, install dotenv and require it in the app.js file:
require("dotenv").config({ path: "./config/.env" });