__noSuchProperty__ poll

[Marcus Lagergren]

I think consistency makes sense, but there might be potential performance implications for modifying the index access. These should probably be investigated.

On Apr 8, 2013, at 7:30 PM, Jim Laskey <jlaskey at me.com> wrote:

> In Nashorn, we introduced __noSuchProperty__ as a means of overriding property search when a property is not declared. Example;
> 
> jjs> var other = { a: 10, b: 20, c: 30 };
> jjs> var obj = { delegate: other, __noSuchProperty__: function(name) { return this.delegate[name]; } } 
> jjs> obj.b;
> 20
> 
> In the current version of Nashorn, we don't call the __noSuchProperty__ handler when accessed using indexing.  However, the question has come up as whether this support should extend to indexing as well; providing symmetry between property access and indexing.  Thus continuing the example;
> 
> jjs> obj["b"];
> 20
> 
> Does this make sense to everyone?
> 
> The only argument I could come up against was using indexing for property testing, as in;
> 
> jjs> if (obj["b"]) print(obj.b);
> 20
> 
> But that argument doesn't hold since there are alternatives;
> 
> jjs> obj.hasProperty("b");
> false
> 
> or;
> 
> jjs> "b" in obj;
> false
> 
> The reason I ask is that we have a bug where megamorphic call sites switch to indexed access (to reduce the number of cases) and thus __noSuchProperty__ is not invoked when it should.
> 
> If we change the behaviour, then we will provide symmetry and address the bug.
> 
> Comments?
> 
> -- Jim
>