lexical for-in/for-of loose end

Jason Orendorff jason.orendorff at gmail.com
Sat Feb 4 08:01:17 PST 2012


On Sat, Feb 4, 2012 at 8:02 AM, Jason Orendorff
<jason.orendorff at gmail.com> wrote:
> On Fri, Feb 3, 2012 at 7:26 PM, Allen Wirfs-Brock <allen at wirfs-brock.com> wrote:
>> Under the scoping rules TC39 has agreed to, the initializer of a let/const is always shadowed by the binding it is initializing[...]
>
> That rule doesn't make sense in this context. There should be either
> one V for the whole loop, or one V per iteration. Having both seems
> perverse.

I just realized—the loop variables have to be visible in the
init-expressions if we want to support this:

    for (let a = getThings(), i = 0, n = a.length; i < n; i++)

This is maybe not the best way to write "for (thing of getThings())",
but people will write it, and so it probably ought to work. I think
this is more important than escaping closures. This means that if such
loops will have per-iteration bindings, they should have an additional
set of bindings just for initialization-time—which seems ugly. Maybe
it's not worth it.

There is also this:

    for (let i = 0; i < n; ) {
        setTimeout(...closure using i...);
        if (shouldAdvance())
            i++;
    }

This will not work no matter what semantics we choose. However,
per-iteration bindings risk encouraging people to hit this problem.

These two issues give me pause.

-j


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