JS Proxy on Clr Object

[martinbu]

Hi,
I am using Jint 3.0.0-beta-2052. I am testing the Js Proxy where I get exceptions if I access a property that exists on a clr object that the proxy was created on. If the property does not exist on the clr object, everything works as expected.
Also everything works fine if the proxy is created on a class created in JS instead of a clr class.

Example

class TestClass
{
     public string Name => "My Name is Test";
}

new Engine()
    .SetValue("T", new TestClass())
    .Execute("""
        const handler = {
            get(target, property, receiver) {

                if (!target[property]) {
                    return (...args) => "Not available";
                }

                // return target[property];
                return Reflect.get(target, property, receiver);
            }
        };

        const p = new Proxy(T, handler);
        const t = p.GetX();
        // const name = p.Name; // When commenting this in, an exception is thrown
    """);

Am I doing something wrong here or did I understand something wrong? Or isn't it supported to use a Proxy on a clr object?

[viruscamp]

I have made a mistake below, It's really a bug.

It's the right behavior in ECMAScript Language Specification.
See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/get#invariants

The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.

All CLR Object Property is non-configurable, and the Name property is obviously non-writable in your code.

[viruscamp]

It's all in JsProxy.Get:

1. Error message must be improved here
2. It called CLR getter twice in TryCallHandler
3. Compared the value in ReferenceEquals will call CLR getter once more
4. Each call to CLR getter returns a new JsValue, so they cannot be ReferenceEqualsed

        /// <summary>
        /// https://tc39.es/ecma262/#sec-proxy-object-internal-methods-and-internal-slots-get-p-receiver
        /// </summary>
        public override JsValue Get(JsValue property, JsValue receiver)
        {
            AssertTargetNotRevoked(property);
            var target = _target;

            if (property == KeyFunctionRevoke || !TryCallHandler(TrapGet, new[] { target, TypeConverter.ToPropertyKey(property), receiver }, out var result))
            {
                return target.Get(property, receiver);
            }

            var targetDesc = target.GetOwnProperty(property);
            if (targetDesc != PropertyDescriptor.Undefined)
            {
                var targetValue = targetDesc.Value;
                if (targetDesc.IsDataDescriptor() && !targetDesc.Configurable && !targetDesc.Writable && !ReferenceEquals(result, targetValue))
                {
                    ExceptionHelper.ThrowTypeError(_engine.Realm, $"'get' on proxy: property '{property}' is a read-only and non-configurable data property on the proxy target but the proxy did not return its actual value (expected '{targetValue}' but got '{result}')");
                }
                if (targetDesc.IsAccessorDescriptor() && !targetDesc.Configurable && (targetDesc.Get ?? Undefined).IsUndefined() && !result.IsUndefined())
                {
                    ExceptionHelper.ThrowTypeError(_engine.Realm, $"'get' on proxy: property '{property}' is a non-configurable accessor property on the proxy target and does not have a getter function, but the trap did not return 'undefined' (got '{result}')");
                }
            }

            return result;
        }

[martinbu]

Thank you for investigation @viruscamp
Should the issue then be reopended?

[viruscamp]

@lahma Please reopen #1643 , I can comfirm it's a bug and I can provide a fix:

1. provide detailed message for exceptions in JsProxy
2. donot use ReferenceEquals on data property value comparing
3. maybe we should mark all Clr Property and Indexer as js accessor property

That's a pure js example which works in V8, but failed in Jint, caused by ReferenceEquals

            let o = Object.defineProperty({}, 'value', {
              configurable: false,
              value: 'oz',
            });
            const handler = {
              get(target, property, receiver) {
                return 'Mozilla'.substring(1,3);
              }
            };
            let p = new Proxy(o, handler);
            let pv = p.value;

[martinbu]

I can confirm that now the example works as expected. Thank you!