The ES6 "fix" is an abomination in my opinion. You're closing over a mutable variable, so you should see the mutations! IMO the real fix would be to write

    for (var i = 0; i < 5; i++) {
        let i_ = i;
        /* use i_ in the closure */
    }

In your ES6 solution if you e.g. increment "i" inside the loop after the closure the closure will see the mutation!

The real cause of confusion is mutation and javascript's scoping rules.