ProxyObject and equality

[marat-gainullin]

I've got a problem with the equals/hashCode contract of ProxyObjectin graalJS. I've implemented ProxyObject for my Java objects like this:

public class MyProxyObject implements ProxyObject
{

	private final Object receiver;

	private MyProxyObject(Object aReceiver){
            receiver = aReceiver;
        }

	@Override
	public boolean equals(Object value)
	{
		Object unwrapped;
		if (value instanceof Value v)
		{
			unwrapped = v.isNull() ? null : v.isProxyObject() ? v.asProxyObject() : v.isHostObject() ? v.asHostObject() : value;
		}
		else
		{
			unwrapped = value;
		}
		return receiver.equals(unwrapped instanceof MyProxyObject ? ((MyProxyObject) unwrapped).unwrap() : unwrapped);
	}

	@Override
	public int hashCode()
	{
		return receiver.hashCode();
	}
}

When I make the bindings like this:

Object sample = new Object();
var bindings = context.getBindings("js");
bindings.putMember("p1", MyProxyObject(sample));
bindings.putMember("p2", MyProxyObject(sample));

I believe that such comparision p1 == p2 in script should evaluate to true because equals and hashCode have been overridden. But the equals method gets never called. This leads to a strange behavior. Let's say that we have a proxy, which returns another proxy for its property. Then we can't get the same property in script multiple times and then compare them as equal. But we expect them to be equal. Can you help with this?

[iamstolis]

This is a misunderstanding, there is no contract when it comes to equals/hashCode of ProxyObject in JavaScript.

ProxyObject is supposed to behave as close to genuine JavaScript object as possible. Note that there is no way for two different JavaScript objects to be equal (in the sense of == operator). Hence, a == b never evaluates to true when a and b are different ProxyObjects.

So, you should either ensure that your MyProxyObject wrappers are unique (per receiver/equality class) or you should implement your own way to compare MyProxyObject instances (for example, by exposing and explicitly using some kind of equality/comparison method as an executable member of MyProxyObject).

[marat-gainullin]

Maybe this is misunderstanding. But the documentation clearly states that ProxyObject is intended to mimic JS Objects. So, if one has two wrappers around the same underlying object it can be expected, that the wrappers should be the same with help of the overridden equals method. Is it worth it to mention this point in the documentation?

[iamstolis]

Let me try to explain that once more from a different angle: there is no notion of object equality (different from object identity) in JavaScript. So, the notion of Java equality (i.e. your equals/hashCode methods) cannot be mapped to JavaScript's behavior of ==. You are basically, asking that it is a !== b but a == b for some instances of MyProxyObject, but this can never happen in JavaScript (when both a and b are objects).