Allen Wirfs-Brock wrote:
On Jun 3, 2015, at 3:46 AM, Leon Arnott wrote:
This reminds me: I feel like the spec should've added
Array.prototype[Symbol.isConcatSpreadable] (value:true,
configurable:false, writable:false), and eliminated the final
IsArray() test from
[22.1.3.11](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-isconcatspreadable).
Would've made the Array#concat() behaviour a little cleaner, in my
opinion.
Unless, perhaps, the point was to leave it open and allow the end-user
to monkey-patch it to false, thus finally "fixing" Array#concat()
after all these years...? Was that the plan?
Exactly! ES6 has to preserve the previous semantics for all existing programs,
including programs that attempted to create Array subclasses in various ad hoc
manners.
@@isConcatSpreadable exists to allow new ES6-level array subclasses to
explicitly opt-out of the legacy implicit spread behavior of concat.
So when one actually _wants_ to concat (as, add elements of the
iterable), it should stop using concat for that and must do something like
var concatenated = first.slice();
concatenated.push(...second, ...third, ...fourth);
?
Allen
Herby
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