NODE_ENV

[isnotafunction]

You could add your local environment to your start script, just so that it does't throw an error if someone tries to run the server locally without running nodemon

Also, I believe you don't need three links here, unless I'm missing something:

let DB_URL = process.env.SKILLSHARE_DB_URL;
if (process.env.NODE_ENV === "test") {
  DB_URL = process.env.TEST_DB_URL;
} else if (process.env.NODE_ENV === "local") {
  DB_URL = process.env.LOCAL_DB_URL;
}

So I think you can only have the condition that tests if you are in the test environment or in your local environment:

let DB_URL = process.env.YOUR_LOCAL_DB_URL;
if (process.env.NODE_ENV === "test") {
  DB_URL = process.env.TEST_DB_URL;
}

Your heroku db link will only be used to build the database but then you will work on your local database while in development environment