How to use custom-UI templates in Forge

[#1]

Hi, after going through the online resources for Forge I have the following doubt. As for this event, there is a need to build a game using the various Rest APIs that Atlassian cloud provides. So, when I opt for a Custom-UI template then it gives a pre-defined template for the particular choice. 

In the guides, it is mentioned that we have to run npm install & npm run build commands after making changes in the code. I want to know that can't I use simple JS (or any other language) to complete the project. Because I have things in my mind, played with the Atlassian APIs but forge is entirely new to me. 

Waiting for someone's reply. 

[Warren Marusiak]

Post questions about Forge here: https://community.developer.atlassian.com/c/forge/45