Object extending Array with Functional Inheritance

Question

Using functional inheritance we can extend objects by passing them as the context of a function call assigning to this.

This doesn't seem to work as I'd expect for the Array constructor.

var ctx = {
    foo: "foo"
};

Array.call(ctx);

ctx -> Object(foo: "foo")

Conversely this does work with other constructor looking functions.

var fakeArrayConstructor = function () {
    this.length = 5;
}

fakeArrayConstructor.call(ctx);

ctx -> Object {foo: "foo", length: 5}

Does the Array constructor not assign some of its properties using this or is there something else going on? I know that a lot of the functionality associated with arrays is stored on Array.prototype however that's not my focus for this exercise.

Array takes an argument as input. So Array.call(null, ctx) works — Mouser
– Mouser, Commented Feb 1, 2015 at 13:14
you seem to confuse the two. functional inheritance, as presented in the article you linked, does not use Array.call. — Bergi
– Bergi, Commented Feb 1, 2015 at 13:16
Sorry but aren't built ins basically bindings to C++ that can do any amount of anything under the hood and don't have to follow any javascript rules, except the explicitly defined ones in the standards..? — Winchestro
– Winchestro, Commented Feb 1, 2015 at 13:25
@Winchestro: Everything in JavaScript is just an exposed binding to some lower-level stuff that conforms to the standard rules. That's basically how every language works… — Bergi
– Bergi, Commented Feb 1, 2015 at 13:36

Ben Aston · Accepted Answer · 2015-02-01 13:57:35Z

1

Functional Inheritance in the linked article is not using call.

Please correct me if I am wrong. The Array function happens to have a peculiar implementation, but that is orthogonal to this question. In order to make functional inheritance work, constructor functions need to be written to support it.

Your Array.call approach is sometimes used to simulate inheritance, but this approach requires methods to be assigned to the instance from directly inside the constructor function, which is probably the exception rather than the rule in most cases.

edited Feb 1, 2015 at 13:57

answered Feb 1, 2015 at 13:14

Ben Aston

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8 Comments

Jamie Dixon Over a year ago

Thanks Ben. Douglas Crockford coined the term "functional inheritance" in his book JavaScript: The Good Parts when discussing inheritance patterns. It's hard to tell from text but am I right to assume you disagree with this in some way? (That's usually what people mean when they refer to something as "so-called"). I think you're right about the internals not working as I'd expect. My comment above about the new keyword mostly covers that. The behavior seems inconsistent to me.

Ben Aston Over a year ago

I have updated my answer with "improved" understanding.

Ben Aston Over a year ago

@JamieDixon No, I have no real opinion on it having only a cursory knowledge of the approach. I used the quotes to indicate that as an idiom, as far as I know, it is not yet mainstream.

Jamie Dixon Over a year ago

In the article linked to the author doesn't use the call function. In Eric Elliot's article on Three Different Kinds of prototypal OO, he does use this approach: ericleads.com/2013/02/… If you're not assigning to this within your constructor, are there other reasons to create objects using new?

Ben Aston Over a year ago

AFAIC remember, new will do three things for you (over and above creating a plain object): 1 - set the receiver (i.e. this), 2 - it will place the .prototype of the constructor function on the prototype chain of the resulting instance, 3 - return the resulting object by default. If you need this functionality, then new is useful. If you don't need this functionality, then new is not useful :)

|

Mark Murray · Accepted Answer · 2015-02-01 13:19:40Z

0

Firstly, functional inheritance allows you to avoid reference to the this keyword all together. The example you linked clearly shows that.

The Array global object is implementation defined but we can assume that all properties are assigned to __proto__ and not this.

answered Feb 1, 2015 at 13:19

Mark Murray

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