There is Rhino behind XPages. And this SO topic
No Array.filter() in Rhino? says, it is out of date.
Edit:
No, it is not. Years ago I read somewhere about it. Now it seems it was not true. According to comment by Dan Sickles (quoting Philippe Riand?) here:
It runs on the server jvm and uses javascript as the application
language. For licensing reasons, IBM wrote their own jvm javascript
engine instead of using Rhino. With Rhino shipping in java 6, they
should be able to ship it in Designer 8.5 (or later). The licensing
problems may have been around the extensions like @Formulas and type
declarations. Classes, modules/namespaces and type declarations are
coming in javascript 2 and even google is helping to get that
implemented in Rhino. I'd hate to see a non-standard, javascript
engine underlying the coolest web development technology in Domino.
Speaking of Rhino, the "most important new feature that is not as
certain to be in 8.5 as XPages" uses Rhino and other jvm scripting
languages on the client. If this makes it into the product, two years
from now most new Notes applications will be written in neither
Lotusscript nor Java. I'll leave it at that.
In fact, there are few topics how to use Rhino in XPages, so with newest Rhino version your code would work. Anyway, my advice is to use Java calls.