It is recommended to use Business Rules to replace the functionalities of JavaScript. Yet, BRs got a lot of limitations, but it can be used for some of the most used aspects like controlling the UI of forms.
Anyway, one of the cool things we did using JS is controlling field visibility and business required levels based on Create Form and Update Form. There is a simple way of achieving the same with BR.
What we do here is checking the CreateOn date. Obviously, we don’t have a value in this field till save the record. Below simple condition shows how the Description field is hidden for Create Form and showing for Update Form.
Caution
Anyway, there is one trick here. Business Rule will only work if CreateOn field is present in the form. What we need to do is, just get the CreateOn into form, but set Visible by Default = No to hide it.