Destructuring syntax infelicity viz. identifier vs. property name

Brendan Eich brendan at mozilla.com
Thu Oct 21 23:15:29 PDT 2010


On Oct 21, 2010, at 10:58 PM, Dominic Cooney wrote:

> The proposed syntax for destructuring objects at <http://wiki.ecmascript.org/doku.php?id=harmony:destructuring> is:
> 
> Pattern ::= "{" (Field ("," Field)* ","?)? "}" 
>         (array destructuring elided)
> Element ::= Pattern | LValue
> Field ::= Identifier (":" Element)?
> LValue ::= <any lvalue expression allowed in a normal assignment expression>
> 
> This misses the opportunity for some parallelism with object literals. For example, I can construct {' 0blah': x} with an object literal, but I can't access the ' 0blah' with object destructuring.

This is just another (still not formal enough, obviously not yet not carefully spec'ed :-|) grammar bug, copied from the ES4 destructuring proposal page. SpiderMonkey and Rhino get it right:

js> var {"0": x} = {"0":42}         
js> x
42
js> var {"0blah": z} = {"0blah":42} 
js> z
42


> Maybe Field ::= Identifier (":" Element)? could be changed to:
> 
> Field ::= PropertyName (":" Element)?
> 
> (using ECMA-262's PropertyName) to permit {' 0blah': x} = ...
> 
> This also allows seeming nonsense like {' 0blah'}, but I'm not sure that is so bad since bindings in *objects* can have names that aren't identifiers, so why not in scopes in general, too?

No, that goes too far. There's no way to name the binding. It's not a property of any object. Again, SpiderMonkey:

js> var {"0blah"} = {"0blah":42}
typein:1: SyntaxError: missing variable name:
typein:1: var {"0blah"} = {"0blah":42}
typein:1: .....^


> So to preserve desugarability you could spell out the alternatives of PropertyName in Field, to require elements with strings and numbers:
> 
> Field ::= Identifier (":" Element)?
>     | StringLiteral ":" Element
>     | NumericLiteral ":" Element

Right, that's the way to go. I'll make sure the wiki page is updated again. Thanks,

/be

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