Subclassing an array and array methods

Axel Rauschmayer axel at
Sat Nov 12 09:38:31 PST 2011

I misunderstood the problem: I thought there was something peculiar about an array instance (the first object in the prototype chain), but it’s about the Array constructor not being invocable as a function (right?).

Wouldn’t it be easier to introduce a generic method Array.prototype.init() that behaves like quirk-free Array.prototype.constructor()?

Use case 1: Create an array subtype:
    let SubArray = Array <| function(...args) {
        super.init(...args); // instead of super.constructor(...args)
Use case 2: Avoid the length pitfall:
    var arr = new Array().init(5); // same as [5]

- Another, possibly stupid, idea: Could Array.prototype.constructor not point to Array, but to a function like Array.prototype.init() – that doesn’t create a new instance and doesn’t have the length quirk?

- One more: Introduce a constant CLEAN
    let SubArray = Array <| function(...args) {
        super.constructor(CLEAN, ...args);
    var arr = new Array(CLEAN, 3); // the same as [3]

- Creating a subtype CleanArray or ExtensibleArray also seems feasible. Engines could optimize internally so that it’s basically the constructor being swapped for a quirk-free implementation.

Whatever the solution, it’s obviously applicable to Date, Error, etc., as well.

> In Smalltalk the default implementation of species is:
> species
>   ^self class
> which is pretty much the moral equivalent of:  this.constructor.
> However, that might introduce backwards compatibility issues for existing code which has constructors but currently always produces Array instances from these functions. 

The problem are Array subtypes that expect slice() et al. to produce instances of Array (and not of themselves) (?)

> function createArraySubclass(proto,...values) {
>     return proto <| [...values].{
>         [Array.derivedArrayKey](){return proto<| [ ]}
>     }
> }

@derivedArrayKey would normally be in the prototype, right?

Dr. Axel Rauschmayer
axel at


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