I've definitely noticed a difference.

I have been developing my current project using IntelliJ, which makes it easy to jump from function call to function definition.

Once I had to do some maintenance on a live copy using vim, where I don't know how to use those features, I decided to gradually redesign things to have fewer levels of nesting for easier maintenance.

The most significant difference in my experience is with dynamically typed languages. I started to write some code in Lua in a simple editor. It worked, but I was very slow, because I made many silly mistakes, mostly typos or trivial errors and those manifested only at runtime, requiring full restart of an application and repeating all actions until that code is run. Then I switched to IDE which highlighted silly errors and allowed autocomplete which eliminated typos and my productivity was few times better. I'm sure that it'll be the same with JavaScript or Python.

My experience with python is that the autocomplete in the REPL is good enough (autocomplete would be better but I don't like the idea of depending on specialized editor configs so it's not worth it in my mind.) If you don't know what fields your classes are then maybe your code isn't organized or documented well or you should be using type hints.

For really messy javascript or things you don't know well autocomplete helps though.

For me, I find that having a good autocomplete makes code easier to read because it makes long names easier to type correctly.

Compare "save-lisp-and-die" vs "s-l-a-d<TAB>". The latter is much easier to type and the former much easeier to read.

funny. we actually have an sdie macro ;)