Array.prototype.slice web-compat issue?

Allen Wirfs-Brock allen at
Tue Sep 10 15:57:38 PDT 2013

On Sep 10, 2013, at 3:24 PM, Rick Waldron wrote:

> On Tue, Sep 10, 2013 at 1:37 PM, Andrea Giammarchi <andrea.giammarchi at> wrote:
> I see one of two problems mentioned in my other topic is already discussed here so I'd like to add this question that will most likely break the web:
>   1. will `[]` produce a pointless instanceof Arguments ?
> No, because you can't construct `new Arguments()` (Arguments is undefined), leaving A undefined at step 15.c, which means step 16 evaluates to true, which leads to 16.a and that produces an Array.

Exactly, as currently spec'ed slice applied to an arguments object or any Array.isArray false produce an instance of Array from the same realm as the slice function.

>   2. will `[]'*'))` produce an instanceof Static NodeList ?
> This is a good question, if NodeList has an internal [[Construct]] it will pass the IsConstructor test and then throw a TypeError exception as soon as 15.c.i occurs. Based on my interpretation of the steps in OrdinaryConstruct, it wouldn't survive. Maybe Allen can clarify?

See %TypedArray%.prototype.slice which in some subtle ways is a cleaner and more general versions of Array slice.

But, in either version I'm not very happy with just assuming that this.constructor(n) produces a new this.constructor instance of size n.  I'm seriously thinking about specify a @@ method on Arrays and other collections that allocates a new instance of a given size.  perhaps Array[@@newSized](n)

In that case, NodeList could define @@newSized and it would work with either version of slice.

> Also, this is the exact pain point case that Array.from was designed to address, but existing code shouldn't break in the interim.

Right, there are several ways in ES6 to force creation of an initialized Array include [...collection] and Array.from

And for existing code, it should just keep working although we have to address the Realm issue.

The other less obvious question is what should happen when old code is combined with new code that uses Array subclassing. My position, is that it will usually just work, but its is essentially newly new code and there may be a few edge cases that the older parts of the code didn't anticipate.  Test is probably required.


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