How to mak changes in new app at code level and what are the functions for event handling and their syntax

Can anyone tell me how to code the app and change the .js and .py file according to our requirement.
What are the functions and how to implement them.

Welcome paras11agarwal

As a developer this may help your learning https://frappe.io/docs/user/en/tutorial

This in particular might address what you seek https://frappe.io/docs/user/en/tutorial/controllers

For pointers that you require, please share details of your requirement - the more specific you are, the better.

Actually we are making ERP for our college but we don’t want to use ERPNext
as it does not provide the flow as we want so either we have to customize
ERPNext or we have to develop our own in frappe.

But we don’t know how what functions to use , their syntax as the tutorial
link does not give full information about the functions.

In that case here’s the Frappe function library https://frappe.io/docs/current/api

And here is the code repository to browse the implementation of Frappe and ERPNext https://github.com/frappe

These may help with your learning and problem solving style, to meet your particular requirements.