Array.prototype.last()

Till Schneidereit till at tillschneidereit.net
Wed May 14 02:37:50 PDT 2014


On Wed, May 14, 2014 at 8:48 AM, Mathias Bynens <mathiasb at opera.com> wrote:

> Previous discussion on this topic:
> http://esdiscuss.org/topic/array-prototype-last
>
> We should look at how existing utility libraries handle this behavior and
> base any proposals on that IMHO. Underscore and Lo-Dash have [`_.first`](
> http://lodash.com/docs#first) and [`_.last`](http://lodash.com/docs#last),
> which both take an optional `callback` parameter, in which case all the
> first/last `n` elements for which `callback` returns a truthy value are
> returned. This seems like a sensible thing to add to the proposal.
>

While I think having .last would be nice, I don't think it's worth the
considerable backwards-compatibility issues. We had to temporarily back out
Array#values from SpiderMonkey until we implement @@unscopable
semantics[1], because we ran into serious real-world breakage. I'm pretty
sure that "last" would break a lot more code, as it's even easier to think
up scenarios where that'd be used as a property name on an array used in
application logic.

At the very least, I think having Array#last be a getter is entirely
untenable: a method would at least just be overwritten in most cases,
whereas a read-only accessor would just (silently, in non-strict code)
fail. A read-write accessor would probably be even worse: it'd silently (in
all code) do something entirely different than it did before.

I do, however, like Axel's proposal[2] of adding .get() and .set() to
Array.prototype, and think that that'd be way less of a compatibility issue.


[1]:
https://mail.mozilla.org/pipermail/es-discuss/2013-July/thread.html#32185
[2]: http://esdiscuss.org/topic/array-prototype-last#content-5
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