[webkit-dev] Optional parameter in IDL and undefined JS value

[Jian Li]

I will pass your good arguments to the spec author. Personally I am not in
strong favor of mimicking Array.slice. I just found the problems we have
during implementation and seek the solution.

On Fri, Apr 22, 2011 at 3:00 PM, Geoffrey Garen <ggaren at apple.com> wrote:

>  I would suggest updating the specification for Blob.slice() so that it
>> doesn't expect passing "undefined" for the second argument to have
>> identical behavior to not passing the argument at all. The "subarray"
>> method in the Typed Array spec [1], which has similar behavior to
>> slice(), doesn't mention the undefined value.
>>
>
> I am afraid that this suggested change might not be preferred since we're
> trying to mimic the behavior of Blob.slice to Array.slice and Array.slice
> does allow passing undefined and treat it as omitting the value.
>
>
> Jian,
>
> Can you clarify why we want Blob.slice to mimic Array.prototype.slice in
> this strange argument quirk?
>
> Let me suggest three reasons why we don't want Blob.slice to mimic
> Array.prototype.slice in this strange argument quirk:
>
> 1. Legacy argument quirks exist for legacy compatibility and nothing else.
> Since Blobs have no legacy requirements, they should not re-introduce the
> mistakes of JavaScript past. There are many legacy argument quirks in
> JavaScript. They often confuse programmers, rather than enabling them. For
> example, new Array(x) creates an array with ToInteger(x) elements in it,
> while new Array(x, y) creates an array with 2 elements in it, x and y. Would
> you mimic that quirk in Blobs if you could?
>
> 2. A Blob is not an Array. Blobs are incompatible with all other Array
> semantics. They don't have a length property. They don't support bracket
> access. They don't support destructive modification. If Blobs were like
> Arrays, you could just use Array.prototype.slice to slice a Blob. But you
> can't, because they're not alike at all.
>
> 3. Your specification doesn't actually conform to the exact semantics of
> all the argument quirks of Array.prototype.slice. There are a number of
> other quirks that you have left out because they are strange and you are not
> familiar with them. I will leave it as a thought experiment for you to go
> and find all the other quirks of Array.prototype.slice that you have missed.
> While you are seeking them out, you can meditate on whether you are making
> your API better or worse.
>
> Geoff
>
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