Comment #2 on issue 4317 by [email protected]: @@isConcatSpreadable is
ignored by Array.prototype.concat when the arguments are arrays
https://code.google.com/p/v8/issues/detail?id=4317
In this version of Chrome, @@isConcatSpreadable is unshipped due to a bug
found with it (see https://code.google.com/p/v8/issues/detail?id=4289). It
can be turned on with --es_staging or --harmony_concat_spreadable, which
can be passed into Chrome as --js_flags=--harmony_concat_spreadable.
With the flag off, Symbol.isConcatSpreadable is undefined. JavaScript's
implicit conversions will take care of the rest for you to make this error
silent and difficult to debug!
8> b = {}
{}
d8> b[Symbol.isConcatSpreadable] = false;
false
d8> b
{undefined: false}
You have a test case there that I can't reproduce:
[].concat(b, new Object).length; // 2 (correct)
For me, this returns 1. Are you sure b was an Array when you executed this
code? Could you paste the whole console log so I can see how you did it?
I'd say this is not a bug; we just need to fix and turn back on
isConcatSpreadable. It's currently off, making it impossible to use.
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