What environment variables does Heroku set by default when we deploy an app?

Heroku sets 4 environment variables automatically and temporarily by default when we deploy an app to Heroku using git push heroku master.

Once the start script is run by Heroku to start our app, Heroku deletes these 4 environment variables. They are only used for the npm install or build purposes.

The 4 environment variables and their values set by Heroku are:-