__noSuchProperty__ poll

[Jim Laskey]

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