More principled implementation

Lazy forwarding proxy.js

LazySet_handlers = {
    has: function(target, name) {
        if (target.inHas) {
            return true;
        }
        target.inHas = true;
        var result = LazySet_installForwarder(target, name);
        target.inHas = false;
        target.hasCalled = true;
        return result;
    },
    get: function(target, name, receiver) {
        if (!LazySet_ensureForwarder(target, name)) {
            return undefined;
        }
        return receiver[name];
    },
    set: function(target, name, val, receiver) {
        if (!LazySet_ensureForwarder(target, name)) {
            return undefined;
        }
        return receiver[name] = val;
    }
}

function LazySet(sources /* : Array<Object> */) {
    var target = {
        sources: sources,
        receiver: this
    }
    this.sources = sources;
    this.__proto__ = Proxy(target, LazySet_handlers);
}

/*
 * SpiderMonkey before FF 25 has a bug (862848) that causes the [[Has]] trap to be 
 * called before the [[Get]] or [[Set]] trap is, if the receiver has a proxy 
 * on its prototype chain. This works around that bug. Without that bug, only the
 * [[Get]] and [[Set]] traps would even be required.
 * (TODO: Test if this works once the bug is fixed.)
 */
function LazySet_ensureForwarder(target, name) {
    if (target.hasCalled) {
        target.hasCalled = false;
        return true;
    } else {
        return LazySet_installForwarder(target, name);
    }
}
function LazySet_installForwarder(target, name) {
    var sources = target.sources;
    for (var i = sources.length; i--;) {
        if (name in sources[i]) {
            Object.defineProperty(target.receiver, name, {
                get: new Function("return this.sources['" + i + "']['" + name + "'];"),
                set: new Function("v", "this.sources['" + i + "']['" + name + "'] = v;")
            });
            return true;
        }
    }
    return false;
}


/* Testing code from here */

var a = {foo: 'bar'};
var b = {zorb: 'zap'};
var c = {cloob: 'barf'};

var rec = new LazySet([a, b, c]);

print("installing forwarders:");
print(rec.foo);
print(rec.zorb);
print(rec.cloob);
print(rec.notthere);
print("using forwarders:");
print(rec.foo);
print(rec.zorb);
print(rec.cloob);
print(rec.notthere);
print("modifying value:");
rec.foo = {toString: function() {return 'baz' + Math.random()}};
print(rec.foo, a.foo, rec.foo === a.foo);