CoffeeScript, prototypal inheritance, and constructors

Question

In CoffeeScript, it seems like the superclasses constructor is not called when you instantiate the subclass.

Is there a way around this?

Here is an example:

class A
    element = null

    constructor: ->
        element = document.createElement "div"

    hide: =>
        element.style.display = "none"

class B extends A
    constructor: ->
        @hide() #error!

I would expect the constructor of A to be called first, then B's constructor. If B then calls the hide method, it should hide the element that was created in A's constructor instead of saying that element is null.

Thanks!

I agree that this seems to be a bug in coffeescript. Have you created an issue for it? — George Mauer
– George Mauer, Commented Jul 5, 2014 at 18:51

robkuz · Accepted Answer · 2013-02-12 20:15:33Z

5

I think you need to call super in the Subclass

class A
    element = null

    constructor: ->
        element = document.createElement "div"

    hide: =>
        element.style.display = "none"

class B extends A
    constructor: ->
        super
        @hide() #error!

answered Feb 12, 2013 at 20:15

robkuz

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5 Comments

tau Over a year ago

Yes, I tried that but it seems like it copies the functions to B, defeating the purpose of the prototype chain. Am I mistaken?

robkuz Over a year ago

I must admit I never have looked into the generated __extends function but it seems to me that it does create the correct prototype chain.

jdb Over a year ago

@tau It does that even if you don't call super. What is your point? All the chains are there: b = new B() A::hello = -> "Hello" alert b.hello()

tau Over a year ago

If I inspect the objects in Chrome, both the sub and superclass have their own hide methods. Is that what it should say? I would expect the hide method to show up only in the superclasses __proto__ property, but I might be misunderstanding Chrome. I thought the functions should only exist in the superclass and not show up under the subclass. Here's an example:

tau Over a year ago

class A constructor: -> test: => alert 'lol' class B extends A constructor: -> b = new B() b.test() console.log b

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