Your C comparison was apples-to-oranges, #ifdef is evaluated at compile
time.
On Fri, Dec 20, 2013 at 8:32 AM, Andreas Rossberg <rossb...@google.com>wrote:
> On 20 December 2013 04:05, Brendan Eich <bren...@mozilla.com> wrote:
> > Andrea Giammarchi wrote:
> >>
> >> why is this not possible, giving the ability to understand through
> typeof
> >> if there is a value or not?
> >>
> >> ```javascript
> >> // defined as const
> >> // reserved in this scope
> >> // but not assigned yet
> >> const WHATEVER;
> >> if (condition) {
> >> // first come, first serves
> >> WHATEVER = 123;
> >> // that's it! const defined for the whole scope
> >> // immutable from now on
> >> } else {
> >> WHATEVER = 456;
> >> }
> >
> >
> > Past JS2/ES4 designs have allowed this, but it requires definite
> assignment
> > analysis and use-before-defining-assignment error checking.
> >
> > In general, such checks can't be static in JS, so the language and VM
> > complexity blow up a bit with runtime checking for an "uninitialized"
> (not
> > same as undefined) sentinel value that must be guarded against by a read
> > barrier where it can't be proven unnecessary.
> >
> > This is pretty obnoxious for implementors, not great for users either
> (did I
> > declare const IMPORTANT; and forget to assign IMPORTANT= in some branch
> of
> > control flow that my tests miss?).
> >
> > It's not in Harmony. We require an initialiser as part of the const
> > declaration syntax. What you are doing here, by many measures, is
> varying a
> > variable from its default (undefined) value to a new value.
> >
> > If you want that variable to stop varying after, and you need it as a
> global
> > (window) object property anyway, use Object.defineProperty to make it
> > non-writable.
> >
> > BTW, the last version your head post gave,
> >
> > const ES6_PROXY = function(){
> >
> > try {
> > new Proxy({},{});
> > return true;
> > } catch(o_O) {
> > return false;
> > }
> > }();
>
> Of course, the problem here is hardly specific to feature detection,
> or const, but simply an instance of the general annoyance induced by
> the old-school statement/expression separation. What you'd really want
> to write is something like
>
> const ES6_PROXY = try new Proxy({}, {}), true catch (_) false;
>
> For ES7 I would like to revive the do-expression proposal (hopefully
> at the next meeting), so that one can at least approximate the above
> with
>
> const ES6_PROXY = do { try { new Proxy({}, {}); true } catch (_) { false
> } };
>
> Of course, semantically the function is equivalent, and a fine
> solution, if a bit verbose.
>
> /Andreas
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>
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