nodemon.json file is only for setting nodemon specific configuration
So for create custom environment variables we can use dotenv package
First , Install dotenv package
npm install dotenv --save
after that create .env file in root and include environment variables as bellows
MONGO_ATLAS_PW=xxxxx
JWT_KEY=secret
Finally, inside your app.js file insert following after your imports.
require('dotenv').config()
Then you can use environment varibale like this
process.env.MONGO_ATLAS_PW
process.env.JWT_KEY