Good points. In hindsight, my complaint was more about things currently being a
bit confusing (one frequently reads that one shouldn’t use instanceof Array)
than about proposing a specific solution. So the comment below helps.
On May 10, 2012, at 4:53 , Allen Wirfs-Brock wrote:
> The reason there will never be a single solution to these issue is that
> desirable behavior varies depending upon what the programmer is trying to
> accomplish. Just using (possibly cross-frame) arrays an an example. What is
> the programmer really asking when they say "is this object an Array". Do the
> need to know whether it has array indexed properties and a length property?
> Do they need to know where it maintains the special Array length invariant?
> Do they need to know whether it implements it has properties that implement
> the array extra methods? Do they need to know that in inherits from the
> build-in array prototype object? Or, do they just need to know whether they
> should serialize the object using [ ] instead of { } notation?
>
> The last one was the situation that jason2.js ran into WRT cross-frame arrays
> and it provided the use case that lead to Array.isArray. But it isn't clear
> that was the best possible solution. Another solution might have been to add
> a serializeAsArray property to Array.prototype. That would work cross frame
> and it would have made it easier to for programmer to define there own
> internal abstractions that would also serialize using [ ] notation.
>
> Whenever anybody says they need to do a type test (or instanceof test) we
> need to ask them: Why? What is it you really need to know. Smalltalkers
> learned fairly early that type/class testing was an anti-pattern. It makes
> code brittle and difficult to fix or extend.
>
> I'm not sure what a cross-frame instanceof test (that didn't always return
> false) would mean. I understand why json2.js thought it needed to do a
> cross-frame isArray test. I would like to understand other real use cases
> where cross-frame type testing is needed. Does anybody have any?
--
Dr. Axel Rauschmayer
[email protected]
home: rauschma.de
twitter: twitter.com/rauschma
blog: 2ality.com
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