This is a simple repo to help you get started building a JS app that can connect to the Yahoo! Fantasy API. It includes an express server (server folder) and a react app (client folder). Simply run npm i inside of each, make the necessary changes to the config and .env files, and you are up and running.
Note: This is meant as a demonstration only to show how you can get started. I've cut a lot of corners and not worried too much about security in setting it up, so please take some time to make sure of your security before publishing anything to a production environment.