isConcatSpreadable set to true

[nzakas]

I may be missing it, but I can't seem to find anything in the spec indicating that @@isConcatSpreadable should be true for arrays.

For @@hasInstance, I noted this entry explaining the default value for functions: http://tc39.github.io/ecma262/#sec-function.prototype-@@hasinstance

I was expected to see something similar for Array.prototype. Is this defined somewhere?

[ljharb]

By my reading, http://tc39.github.io/ecma262/#sec-isconcatspreadable indicates that if Symbol.isConcatSpreadable is not present, then it falls back to IsArray - and also that it is not initially present on any value.

My understanding is that the well-known symbol is provided as an opt-out mechanism for Array subclasses who do not want the spreading behavior of concat, to ensure that legacy code paths continue to work with Arrays.

[ljharb]

By my reading, http://tc39.github.io/ecma262/#sec-isconcatspreadable indicates that if Symbol.isConcatSpreadable is not present, then it falls back to IsArray - and also that it is not initially present on any value.

My understanding is that the well-known symbol is provided as an opt-out mechanism for Array subclasses who do not want the spreading behavior of concat, to ensure that legacy code paths continue to work with Arrays.

[nzakas]

Ah! Totally missed that. Thanks.

[nzakas]

Ah! Totally missed that. Thanks.