What is the use of overriding hasMember() inside JSObject, AbstractJSObject?

Question

I understand that this helper method can be called from a Java code to check if your Java/JS Object has a property you are looking for but i would like to know if this is called by the Nashorn Engine while we use this JSObject/AbstractJSObject implementation in a JavaScript code.

I am aware of the fact that doing a . inside JavaScript will in turn invoke the Java method .getMember()

Answer 1

If "in" operator in used in JavaScript (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in) on a JSObject instance, Nashorn will call hasMember method on that JSObject.

Example code:

import javax.script.*;
import jdk.nashorn.api.scripting.*;

public class Main {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager m = new ScriptEngineManager();
        ScriptEngine e = m.getEngineByName("nashorn");
        e.put("obj", new AbstractJSObject() {
            @Override
            public boolean hasMember(String name) {
                System.out.println("hasMember called for " + name);
                return false;
            }
        });
        // in operator triggers hasMember call on JSObject instance
        e.eval("if ('foo' in obj) print('yes')");
    }
}

The output from the above program looks like:

hasMember called for foo