Is it possible to override the equivalence comparison in Javascript?

The closest I have gotten to a solution is by defining the valueOf function and invoking valueOf with a plus in front of the object.

This works.

equal(+x == +y, true);

But this fails.

equal(x == y, true, why does this fail.);

Here are my test cases.

var Obj = function (val) {

    this.value = val;

};

Obj.prototype.toString = function () {

    return this.value;

};

Obj.prototype.valueOf = function () {

    return this.value;

};

var x = new Obj(42);

var y = new Obj(42);

var z = new Obj(10);

test(Comparing custom objects, function () {

    equal(x >= y, true);

    equal(x <= y, true);

    equal(x >= z, true);

    equal(y >= z, true);

    equal(x.toString(), y.toString());

    equal(+x == +y, true);

    equal(x == y, true, why does this fails.);

});

Demo here: http://jsfiddle.net/tWyHg/5/