@@can.onValue( handler(newValue) )

The @@can.onValue symbol points to a function that registers handler to be called back with the new value of the object when it changes.

var obj = function(value) {
    if(arguments.length >= 1) {
        obj.currentValue = value;
        obj.handlers.forEach(function(handler){
            handler.call(obj, value);
        });
    } else {
        return obj.currentValue;
    }
};

obj[canSymbol.for("can.onValue")] = function(handler){
    if(!obj.handlers) {
        obj.handlers = [];
    }
    obj.handlers.push(handler);
}