Passing Java lists into JS

Tim Fox timvolpe at
Thu Nov 27 09:41:52 UTC 2014

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<>();
      return list;

I call this from JavaScript:

var 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?


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