Extending an Array properly, keeping the instance of subclass

Question

I've written a class trying to extend the native Javascript Array class with a custom class, let's call it MyClass. This is basically what it looks like:

class MyClass extends Array

  constructor: (obj) -> @push.apply @, obj

  first: -> @slice 0, 1

Instantiating the class is no problem. Running this in the console:

var myInstance = new MyClass(["1", "2"])
> ["1", "2"]
myInstance instanceof MyClass
> true
myInstance instanceof Array
> true

works as exptected.

The problem is that if I run:

myInstance.first()
> ["1"] // as expected
myInstance.first() instanceof MyClass
> false // not expected
myInstance.first() instanceof Array
> true

the returned value is no longer an instance of MyClass.

I've also tried @__proto__.first = @first in the constructor function and first: -> @slice.call @, 0, 1. But with no success.

Why doesn't myInstance.first() instanceof MyClass return true?

I don't think extending Array will ever work properly (stackoverflow.com/q/3261587/479863), why are you trying to do it? — mu is too short
– mu is too short, Commented Mar 18, 2014 at 18:57
Basically this is only experimental, I wanted to see if it's possible. Instead of polluting the Array class, I wanted to rather augment my own class. — Johan Dettmar
– Johan Dettmar, Commented Mar 19, 2014 at 9:47

Bergi · Accepted Answer · 2014-03-18 20:26:56Z

1

Why doesn't myInstance.first() instanceof MyClass return true?

Because first calls slice, and Array.prototype.slice does always return an Array. You will need to overwrite it with a method that wraps it in a MyClass again:

class MyClass extends Array

  constructor: (obj) -> @push.apply @, obj

  slice: () -> new MyClass super
  splice: () -> new MyClass super
  concat: () -> new MyClass super
  filter: () -> new MyClass super
  map: () -> new MyClass super

  first: -> @slice 0, 1

And notice that subclassing Array does not work.

answered Mar 18, 2014 at 20:26

Bergi

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6 Comments

Johan Dettmar Over a year ago

Thanks! That solves the issue! However regarding memory footprint, wouldn't new MyClass super consume a bit more memory? Also, thanks for the article link!

Bergi Over a year ago

Sure it will. However, you want more than an Array :-)

Johan Dettmar Over a year ago

Yep, I totally get that. My questions should have been phrased like this; is there a way to create a new instance once, instead of each time I use for example slice()?

Bergi Over a year ago

I don't understand, the purpose of slice is to return a different, new instance? Do you want it to mutate your instance?

Bergi Over a year ago

No, it doesn't modify. All the other array methods that do mutate (like sort, push, shift, …) work on MyClass instances without problems.

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