I finally get it. Thankyou for taking the time to explain this, I appreciate it.

I just don't understand how the code doesn't fail when the map looks to be created after the code before is executed...

The confusion seems to come from misreading the body of the arrow function:

foo((map, v) => map.set('foo', v), new Map());

In the code above, map.set('foo', v) is not called before new Map() because it's inside the anonymous function passed to foo.

Maybe rewriting like the following will help you understand:

const f = (map, v) => map.set('foo', v);
foo(f, new Map());

The best answer, well done.

Yeah I read that, most of it makes sense but that one thing i requested I just don't understand how the code doesn't fail when the map looks to be created after the code before is executed...

check down three posts!

Ah I get it! Much apreciated!!

@Souzooka & cave.on

My solution used the .filter() method also. I'm still learning, do you think in a working environment your solutions using the for loop would be better practice?

Appreciate any feedback!

That's the good stuff.

@Blind4Basics & @B1ts Thankyou both, much appreciated!