Obsolete Application.ExternalCall

I use Application.ExternalCall in my WebGL application. But it is obsolete and I do not quite understand how to replace it with new methods. The documentation says that you need to add javascript code directly to the source code of the unit, but can this be avoided? Suppose now I have scripts written in the same html file in which WEBGL is loaded.

I could write a function in index.html, call it in Unity using Application.ExternalCall and then I could change the function without changing their Unity source, is it possible now with the new system?