Graal embedded javascript in java, how to call map on list/array from java? Is it possible?

Question

I am playing with Graal for running javascript as a guest language, and would like to know if there is a way to use javascript Array.map functionality on a host (Java) object or proxy. Demo Kotlin code follows, but should be close enough to Java code.

fun main() {
    val context = Context.newBuilder().build()
    val javaOutputList = mutableListOf<Integer>()
    val javaList = listOf(2, 2, 3, 4, 5)
    val proxyJavaList = ProxyArray.fromList(javaList)

    context.polyglotBindings.apply {
        putMember("javaOutputList", javaOutputList)
        putMember("javaList", javaList)
        putMember("proxyJavaList", proxyJavaList)
    }

    val script = """
        var javaOutputList = Polyglot.import('javaOutputList');
        var javaList = Polyglot.import('javaList');
        var proxyJavaList = Polyglot.import('proxyJavaList');

        var abc = [1, 2, 3];
        abc.forEach(x => javaOutputList.add(x));      // WORKS

        //abc.map(x => x + 1)             // WORKS
        //javaList.map(x => x + 1)        // DOES NOT WORK (map not a method on list)
        proxyJavaList.map(x => x + 1)     // DOES NOT WORK (message not supported: INVOKE)
    """.trimIndent()

    val result = context.eval("js", script)

    val resultList = result.`as`(List::class.java)
    println("result: $resultList")
    println("javaOutputList: $javaOutputList")

}

Using ProxyArray looked the most promising to me, but I still couldn't get it to work. Is this functionality expected to be supported?

EDIT: with the accepted answer the code works, here is the change for the interested:

    val context = Context.newBuilder()
            //.allowExperimentalOptions(true)  // doesn't seem to be needed
            .option("js.experimental-foreign-object-prototype", "true")
            .build()

Answer 1

The root of the problem is that array-like non-JavaScript objects do not have Array.prototype on their prototype chain by default. So, Array.prototype.map is not accessible using javaList.map/proxyJavaList.map syntax.

You can either invoke Array.prototype.map directly like Array.prototype.map.call(javaList, x => x+1) or you can use an experimental option js.experimental-foreign-object-prototype=true (that we added recently) that adds Array.prototype on the prototype chain of all array-like objects. javaList.map/proxyJavaList.map will be available then.