UnhandledPromiseRejectionWarning: MongooseError: The `uri` parameter to `openUri()` must be a string, code example

Example: unhandledpromiserejectionwarning: mongooseerror: the `uri` parameter to `openuri()` must be a string, got "undefined".

I think you miss importing env file.

require('dotenv').config({ path: 'ENV_FILENAME' });