Which Single Resource Has Most Affected How You Code?

Sure. Even just using pure functions can change the way you write things.

Whenever I get to write a function from scratch I end up with something like this.

function big_function() {
  const data = get_data();

  const errors = validate(data);
  if(errors.length) {
    show_errors(errors);
    return;
  }

  const clean_data = some_process(data);
  const saved = save_stuff(clean_data);

  if(saved == false) {
    show_errors(['oops, something went wrong']);
    return;
  }

  show_message('we good');
}

I have a 'big function' that coordinates a bunch of 'little functions.'

The things that can have an effect on the outside world get their own functions and most of the time don't process anything, they just show/save/send whatever you throw at them.

Almost everything ends up being a two step process, handle the data and using the data.

The most important thing is to make the little functions as predictable as possible. Given the same inputs should always give you the same result.

Nope, never before reading that and not once since, save a little Clojure a few years ago. It's a book about programming, and more specifically recursion, not necessarily Scheme. Scheme is a very minimal language and almost functions more as a formal notation for computation than anything else.