Passing Java lists into JS

[A. Sundararajan]

List return values from Java are not converted to JS Arrays. But usual 
'indexing' to get elements and ".length" to get size work (later added 
in 8u20 or 8u40 - need to check). And usual for..in and for..each..in 
(extension) work on Lists. Rational is to avoid wrapping Lists as 
another Script object.


-Sundar

Tim Fox wrote:
> Hello again,
>
> I am a bit confused about how Lists passed from Java into JS are 
> converted.
>
> I have a Java class as follows:
>
> public class SomeClass {
>
>    public List<String> provideList() {
>      List<String> list = new ArrayList<>();
>      list.add("foo");
>      list.add("bar");
>      return list;
>    }
> }
>
> I call this from JavaScript:
>
> var io = Packages.io;
> var someObject = new io.vertx.scratchpad.SomeClass();
>
> var arr = someObject.provideList();
>
> console.log(arr[0]); // prints: foo
> console.log(typeof arr.length); // undefined
> console.log(arr instanceof Array); // false
>
> I was under the impression that Nashorn automatically converted Java 
> lists passed into JS into JS Arrays.
>
> The object arr returned in some ways resembles a JavaScript array - 
> the operators [] and []= work on it, however it doesn't have the array 
> property "length" and it's not an instanceof Array.
>
> Can anyone clarify to me what this object is? Any reason why Nashorn 
> doesn't just wrap it as a real JS Array?
>
> Cheers