On Tue, Sep 11, 2012 at 11:37 AM, Dmitry Soshnikov < [email protected]> wrote:
> On Tue, Sep 11, 2012 at 1:33 AM, Brendan Eich <[email protected]> wrote: > >> >> If you really want the Pythonic default parameter rules (evaluation in >> definition context) then you need to address the bad case Jason showed: >> >> function foo(arg = []) { >> arg.push('strange'); >> return arg; >> } >> foo(); // ['strange'] >> foo(); // ['strange', 'strange'] >> >> This is a side channel and a big footgun. ES6 default parameters as >> proposed avoid it. >> >> > Sure, and as I mentioned in my first message -- ES may eval defaults every > time at activation, not only once as Python does. But -- still in the outer > scope (in the foo.[[Scope]] in your example). > > function foo(arg = <expression>) { ... } > > evaluates the <expression> every time at activation as: > > 1. If expression is a literal value, create a *new* value corresponding to > literal. Assign to the parameter. Return. > OK, now I see that this defers to the activation time *only*, which probably doesn't make much sense in evaluation in only parent scope. Sorry for confusion, yes, in this case evaluation including activation frame fits better. And I also see now Allen's example with generators, which, if to accept desugaring you noted (with if (== void 0) then assign) may be an issue if creation of the generator should not trigger the activation until the "next" call. Dmitry
_______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
