When to use "is a" or "has a"?

Question

In my node.js application, I have a queue class which has push and pop methods and a data property. I have an Event class which handles an event and pushes it on to the queue.

If I think object oriented, is the relationship between queue and event better modeled as "is a" (inheritance) or "has a" (composition)?

First approach:

var util = require('util'),
    array = [];

function Q(source){
    this.data = source;
}

Q.prototype.push = function(x){
    this.data.push(x);
};

Q.prototype.show = function(){
    console.log(this.data);
};

function Event(y){
    Q.call(this, y);
}

util.inherits(Event,Q);

Event.prototype.trigger = function(bb){
 //some logic
 this.push(bb);
};

var x = new Event(array);
x.trigger('hi');
x.show();

Second approach :

var array = [];
function Q(source){
    this.data = source;
}

Q.prototype.push = function(x){
    this.data.push(x);
};

Q.prototype.show = function(){
    console.log(this.data);
};

function Event(y){
    this.arr = new Q(y);
}

Event.prototype.trigger = function(bb){
    //some logic
    this.arr.push(bb);
};

var x = new Event(array);
x.trigger('hi');
x.show();

possible duplicate of [Are there any OO-principles that are practically applicable for Javascript?](http://programmers.stackexchange.com/questions/180585/are-there-any-oo-principles-that-are-practically-applicable-for-javascript) — gnat, Jan 12 '16 at 11:10
@gnat thats absoloutly not duplicate of [Are there any OO-principles that are practically applicable for Javascript](http://programmers.stackexchange.com/questions/180585/are-there-any-oo-principles-that-are-practically-applicable-for-javascript). you just wast my time by reading that long question and answer. my question is about pros and cons of `is a` and `has a` in the above situation. — Jafar Rasooli, Jan 12 '16 at 11:40
recommended reading: **[What is the problem with “Pros and Cons”?](http://meta.programmers.stackexchange.com/q/6758/31260)** — gnat, Jan 12 '16 at 11:45
You need to define what "better" means to you, in this particular case. What are you most concerned with? Least concerned with? — Thomas Owens, Jan 12 '16 at 13:34

score 4 · Accepted Answer · answered Jan 12 '16 at 13:49

Conceptually, inheritance (is-a) is a specialization of composition (has-a). (from Reg Braithwaite) Both accomplish the same thing (which is why you can ask a question like this), but inheritance lacks the flexibility of composition. Inheritance makes your code rigid.

From a practical view, this doesn't matter as much in JavaScript since JavaScript isn't limited to single inheritance. What we're calling inheritance is actually some prototype patching and function delegation. There's nothing stopping us from creating a mixin from several "types". This has more in common with Go's type composition than Java or C#'s inheritance model. It doesn't make your code as rigid.

Still, it does seem strange to say an Event is-a Queue. Is it really? It seems more likely that an Event is a model of some action in your app and it uses (has-a) a Queue to manage its workflow. If an Event is truly a Queue, I should be able to use it anywhere I use a Queue without a second thought:

var q = new Event();
q.push("hi");
q.pop();

This doesn't seem right to me. It's a stretch. This is a perfect place to avoid inheritance in favor of composition.

When to use "is a" or "has a"?

1 Answers1