Javascript inheritance and privileged functions

Question

Is it possible to inherit privileged methods in Javascript? In the below, Widget successfully inherits the D function, but not subscribe. Changing the second line in inherit to f.prototype = new base(); seems to work, but I know that's bad for all sorts of reasons. Is there a clean way to do this, or do I have to make everything public? This answer seems to imply that I have to make the methods public (attach to prototype) but I'd like to ask directly.

    function EventRaiser() {
        var events = {};
        this.subscribe = function(key, func) { /* elided */ };
    }
    EventRaiser.prototype.D = function() { alert("D"); };

    function Widget() { }

    function Inherit(sub, base) {
        function f() { }
        f.prototype = base.prototype;
        sub.prototype = new f();
        sub.prototype.constructor = sub;
    }

    Inherit(Widget, EventRaiser);

Raynos · Accepted Answer · 2011-09-21 17:20:59Z

1

this.subscribe = function(key, func) { /* elided */ };

here your adding a method to the current thisContext.

Inherit(Widget, EventRaiser)

Here your saling the prototype Widget should consume the prototype EventRaiser.

The best you can do is to not mix this.x with prototype.y

Or you can call EventRaiser.call(this) inside function Widget() { } but that's bad style.

If your going to use an inheritance pattern I would recommend you use Object.create & pd :

// prototype
var EventRaiser = {
    D: function() { alert("D"); },
    subscribe: function(key, func) { ... }
};
// factory
var eventRaiser = function _eventRaiser(proto) {
    proto = proto || EventRaiser;
    return Object.create(proto, pd({
        events: {}
    }));  
};

// prototype
var Widget = {
    ...    
};
// factory
var widget = function _widget() {
    var o = eventRaiser(pd.merge(Widget, EventRaiser));
    Object.defineProperties(o, pd({
        ... 
    });
    return o;
};

Or if you insist Widget should inherit from EventRaiser

var Widget = Object.create(EventRaiser, pd({
    ...
});
var widget = function _widget() {
    var o = eventRaiser(Widget);
    Object.defineProperties(o, pd({
        ...
    });
    return o;
}

The reasons for recommending this pattern is a clear seperation of the prototype and the factory. This allows you to interact with the prototype without handling the factory. With the use of the new keyword you muddy those waters (as shown in your code) and you also tend to hack this around.

The above code also doesn't look elegant. This means that your realy aught to look for a different pattern. For example EventEmitter from node.js has an explicit check for the this._events object which makes the factory code more elegant.

edited Sep 21, 2011 at 17:20

answered Sep 21, 2011 at 16:23

Raynos

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2 Comments

Adam Rackis Over a year ago

EventRaiser.call(this) is what I thought I needed. Why is that bad style? Chaining to your base ctor is common practice in OO Languages, and even this JS sample does it js-bits.blogspot.com/2010/08/…

Raynos Over a year ago

@AdamRackis There is no notion of base constructor. Your emulating classical OO and Java in JavaScript. That's bad style

Alnitak · Accepted Answer · 2011-09-21 16:24:08Z

1

Your privileged method this.subscribe is only ever attached to an instance of EventRaiser.

This should be obvious when you consider that this line:

this.subscribe = function(...) { } ;

is only ever executed when EventRaiser() is called.

If you don't create an EventRaiser object that property is simply never there to be inherited from.

answered Sep 21, 2011 at 16:24

Alnitak

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