On Nov 5, 2011, at 9:38 AM, Dmitry Soshnikov wrote:
> On 05.11.2011 20:28, Allen Wirfs-Brock wrote:
>> In the following declaration, what should be the value of z?
>>
>> let [z,y,z] = {0:0, 1:1, length: 2, 2:2};
>>
>> should it be be 2 or undefined
>>
>> undefined might be reasonable because it is an array pattern, and the source
>> object is only has two "array-like" elements
>> 2 might be reasonable because the source object actually has a property
>> named "2"
>>
>> Which alternative will be least surprising to JS programmers?
>>
>
> Since arrays may participate in the same sense as objects, e.g. for-in loop
> or some generic methods (e.g. o = {push: [].push}; o.push(1); o.length is
> created and is 1), the example can be (by logic) correct.
The question isn't whether or not the example is correct. It is already
determined that it is valid syntax. The question is about semantics. Does
"array" destructuring of a "array-like" object controlled by the "length"
property of the object?
>
> Besides, since the destructuring pattern matching isn't strict (i.e. it's
> used only for exactly for destructuring), the example also is OK since it
> doesn't say "we match an array", but we only destructure a complex structure
> on some less complex parts, and if the pattern on the left can manage it,
> it's OK.
Personally, I dislike calling this style of destructuring (as defined in JS)
"pattern matching" because that makes it sound like it is doing much more than
it really is.
>
> If of course there had been the case of strict pattern matching (like e.g. in
> Erlang in this case), then it should be an error.
Allen
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