How to identify arguments object as Value?

[provegard]

I'm trying to migrate a big Nashorn application to GraalJS. One of the things the code does is convert a JavaScript value to a Scala/Java value. An Arguments object is detected and converted to an array in Nashorn like this:

case a: ScriptObjectMirror if a.getClassName == "Arguments" => extractArray(a)

I haven't succeeded in testing if a Value is Arguments. My test method looks like this:

public void acceptValue(Value v) {
    System.out.println("value = " + v);
    System.out.println("meta = " + v.getMetaObject());
    System.out.println("meta:type = " + v.getMetaObject().getMember("type"));
    System.out.println("meta:className = " + v.getMetaObject().getMember("className"));
    System.out.println("meta:description = " + v.getMetaObject().getMember("description"));
}

And I'm calling it like this:

context.eval("js", "(function () { main.acceptValue(arguments); })('a', 'b', 'c')");

This is the printout:

value = {0: "a", 1: "b", 2: "c", length: 3, callee: {...}, Symbol(Symbol.iterator): {...}}
meta = Object
meta:type = object
meta:className = Object
meta:description = {0: "a", 1: "b", 2: "c", length: 3, callee: {...}, Symbol(Symbol.iterator): {...}}

Sadly, neither type nor className are useful here.

Any suggestions?

[chumer]

I don't think you can detect whether an array is in fact the arguments array, but you may detect that it is an array.

This is how you can check for an array:

    public static void main(String[] args) {
        try(Context ctx = Context.newBuilder().arguments("js", new String[] {"arg0", "arg1"}).build()) {
            Value arguments = ctx.eval("js", "arguments");
            System.out.println("getMetaObject().toString(): " + arguments.getMetaObject().toString());
            System.out.println("hasArrayElements(): " + arguments.hasArrayElements());
            System.out.println("string array(): " + Arrays.toString(arguments.as(String[].class)));
        }
    }

This prints:

getMetaObject().toString(): Array
hasArrayElements(): true
string array(): [arg0, arg1]

I'd recommend to use hasArrayElements as it is language agnostic.

Does this help? (feel free to close if it does)

[provegard]

What I'm after is the arguments variable that exists inside a JavaScript function. I modified your code:

        try(Context ctx = Context.create("js")) {
            Value arguments = ctx.eval("js", "(function () { return arguments; })('a', 'b', 'c')");
            System.out.println("getMetaObject().toString(): " + arguments.getMetaObject().toString());
            System.out.println("hasArrayElements(): " + arguments.hasArrayElements());
            System.out.println("string array(): " + Arrays.toString(arguments.as(String[].class)));
        }

It prints:

getMetaObject().toString(): Object
hasArrayElements(): false
Exception in thread "main" java.lang.ClassCastException: Cannot convert '{0: "a", 1: "b", 2: "c", length: 3, callee: {...}, Symbol(Symbol.iterator): {...}}'(language: JavaScript, type: Object) to Java type 'java.lang.String[]': Value must have array elements.

[provegard]

I tested another variant:

        try(Context ctx = Context.create("js")) {
            Value arguments = ctx.eval("js", "(function () { return arguments; })('a', 'b', 'c')");
            ctx.getBindings("js").putMember("obj", arguments);
            System.out.println(ctx.eval("js", "Object.prototype.toString.call(obj)"));
        }

And it prints:

[object Arguments]

Hooray! :) Can I obtain that value in Java land without executing JS?

[chumer]

@woess Why does the arguments object not return true for HAS_SIZE? Is it not an array? Seems like a bug to me.

[woess]

@chumer technically, an arguments object is not a JS array (Array.isArray(arguments) returns false). But arguably, it has a size and array elements, so HAS_SIZE should probably return true...

[woess]

@provegard

Can I obtain that value in Java land without executing JS?

I'm afraid that's the only reliable way to identify if something is an arguments object currently. You can of course do something like this:

Value isArguments = ctx.eval("js", "(function(obj) {return Object.prototype.toString.call(obj) === '[object Arguments]';})");

boolean isArguments(Value obj) {
  return isArguments.execute(obj).asBoolean();
}

[provegard]

@woess That is what I'm doing now, and it works well.

Feel free to close the issue unless you want to keep it open for the HAS_SIZE thing.

[woess]

As of 8423934, HAS_SIZE/hasArrayElements() returns true for arguments objects.