I have made a website meant to me used by only one person, so I want to dynamically write to .env file on Heroku without it resting,
because this is meant only for one person. I don’t want to deal with a database.
Something like this:
require(`dotenv`).config();
console.log(process.env.MYVAL); // Not my value
process.env.MYVAL = "MYVAL"
console.log(process.env.MYVAL); // MYVAL
You can set the environment variables in the settings tab on your Heroku dashboard and also using the command line. Please check the following documentation to get more information.
You need to persist data (even if it is a single value). Therefore you should not write to Heroku file system nor storing it in environment variables (Heroku configuration variables).
I understand using a database could be not worth it, and in this case I would use an external file storage (Amazon S3, Dropbox, and even using GitHub private repository).
On Files on Heroku you can see some options and (Python) code.