proxy as argument would help here

gistfile1.js

function makeProxyWhichCallOnDelete(farg){
    var target = {}; // Important: the constructor was not initialized with the target

    return new Proxy(target, {
        delete: function(target, name){
            farg(target); // no access to the proxy identity!
        }
    });
}

(function(){
    var values = new WeakMap();

    function f(o){
        console.log(values.get(o));
    }

    var p = makeProxyWhichCallOnDelete(f);
        p2 = makeProxyWhichCallOnDelete(f);
    values.set(p, 1); // Setting the value with the proxy, not the target!
    values.set(p2, 2);

    p.prop = "yo";
    delete p.prop; // trap called
    // f will receive the target as argument while it has values stored
    // with proxy identities
})();