On 06.05.2010 10:06, Biju wrote:
var lastItem =
myObject.anArray[ myObject.anArray.length - 1 ];
myObject.anArray[ myObject.anArray.length - 2 ] = "foo";
a = myObject.anArray; a[a.length - 1];
a[a.length - 2] = "foo";
What is the problem allowing following instead of above?
a = myObject.anArray; a[-1];
a[-2] = "foo";
The problem is that currently ES has only concept a "property". And
despite the fact that spec defines concept of an "array index", it still
"virtual" concept and is only for ability to make some checks or actions
(e.g. update "length" property with overloaded [[Put]] method). Still,
these properties are only strings. And because of there is no semantic
stratification between the dot and the bracket property accessor
notations (except the the former is used only when property name is a
valid identifier and in advance known), i.e. they are not divided into
"keys" and "array indexes" for the bracket notation, and "properties"
and "methods" for the dot notation, currently there is no ability to
distinguish whether "-2" is a simple (or even inherited) property or
means "length - 2" index.
Of course, there could be overloaded [[Get]] semantics for arrays, but
this is not in ES5 (who's spec is already published and it's being
implemented). The other questions should be clarified then, such as if
there is e.g. Object.prototype[-3] = 10; and let a = [1, 2]; then what
should return a[-3] if a[-2] returns 1. I.e. should be semantics of the
arrays [[Get]] be completely overloaded and do not inherit such property
names, or only partly (the last case will cause ambiguity).
But in general, bracket slicing (used widely in some languages) is more
convenient than explicit "slice" method which does the same. But notice,
that in contrast with ES in those languages there is stratification in
semantics of the dot and bracket notations.
In contrast, proposed "last" method seems also ambiguous in case of
setting and looks a bit ugly. That taking into account that there are
method names with "get" semantics. So the setter "last(-index, value)"
looks odd. So I don't think this proposal is very required. For
convenience as maximum you can implement only a getter in the
Array.prototype for just a.last.
It would be incompatible, since a[-1] means the same thing for arrays
as for objects: the property named "-1".
And I dont think any body depending on such code now,
ie expecting "-1" as property name in Array
I have had once upon a time ;)
Dmitry.
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