No, that was to underline it is possible to define this twice static int const NAME =
On Fri, Dec 20, 2013 at 8:49 AM, J B <[email protected]> wrote: > Your C comparison was apples-to-oranges, #ifdef is evaluated at compile > time. > > > On Fri, Dec 20, 2013 at 8:32 AM, Andreas Rossberg <[email protected]>wrote: > >> On 20 December 2013 04:05, Brendan Eich <[email protected]> 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 >> _______________________________________________ >> es-discuss mailing list >> [email protected] >> https://mail.mozilla.org/listinfo/es-discuss >> > > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss > >
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