lexical for-in/for-of loose end

Brendan Eich brendan at mozilla.org
Tue Jan 31 08:36:52 PST 2012

> Allen Wirfs-Brock <mailto:allen at wirfs-brock.com>
> January 31, 2012 8:28 AM
> On Jan 30, 2012, at 6:38 PM, Brendan Eich wrote:
>> No! We have a standing agreement to get rid of the awful, lazy-grammar-reuse error in ES1 that allows an initializer in for(var x=i in o). We do not want this at all for 'let' in either for-in or for-of.
> Fine by me, WRT let/const
> We haven't done a very good job of capturing such agreements.  I don't think this one is reflected on the wiki proposals...

You're right, I thought I'd recorded it. Sorry, fixed:

>> I think we should break unconditionally and forbid =i in for(var x=i in o) too, but that is a separate issue. This botch in grammar factoring is a (bad) sunk cost that has zero bearing on the fresh let binding per iteration idea. It's terrible anti-precedent. Just say no.
> While I think the language would be better without it.  I don't really see how we can justify such a breaking change.  Heck, I can even think of a plausible use:
> function hasEnumerableProperties (obj) {
>     for (var key = false in obj);
>     return key!==false
> }
Yes, unlike 'let' the binding persists after the loop. We can live with 
this, my hope was that we could also make it an early error and survive 
because as Arv suggests, it's probably used only in testsuites. But you 
never know, and the Web tends to tile all executable surfaces and test 
all features. Let's see how easy it is to split the 'var' and 
'let/const' cases...

But you could make a twisted argument for an effect-ful 'let' 
initialization expression also being "useful" in this way. We mustn't go 


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