This is why I'm really excited about htmx [1]. No need to write isomorphic javascript at all. You can still use server side templates but have interactive web pages.
When you start using htmx, you raise your eyebrows and think - hmmm this could be something interesting. When you use it for many months, you then open your eyes very wide and think - this is something special! In hindsight is so damn obvious, why didn't it happen much earlier?!?!