On Jan 21, 2015, at 6:09 PM, Mark S. Miller wrote:
>
>
> On Wed, Jan 21, 2015 at 6:01 PM, Allen Wirfs-Brock <[email protected]>
> wrote:
>
> On Jan 21, 2015, at 2:09 PM, Mark S. Miller wrote:
>
>> // old ES5 code
>>
>> function f(allegedDate) {
>> if (({}).toString.call(allegedDate) === "[object Date]") {
>> JSON.stringify(allegedDate); // "[]" impossible in ES5
actually it isn't impossible. allegedDate could have a `toJSON` method that
returns `"[]`
This is a pretty good example of why this sort of brand check is so
problematic. It is based upon and supports the misperception that such a
branded object will have all of the specified initial characteristics of the
correspond built-in.
>> Array.isArray(allegedDate); // true impossible in ES5
>> Date.prototype.getYear.call(allegedDate); // error impossible in ES5
>> }
>> }
>>
>>
>> // new ES6 code
>>
>> const fakeDate = [];
>> const defProp = Object.defineProperty;
>> defProp(fakeDate, Symbol.toStringTag, { value: "[object Date]" });
>> f(fakeDate); // all ES5 impossible behaviors happen
>
> not quite. The last of your tests (getYear) will still fail because fakeDate
> is not internally branded as a date.
>
> Ignoring the unintended Annex B issue, that failure supports my point. In ES5
> we could not have failed at this point, because this point is only reached if
> allegedDate is a genuine Date.
>
>
>
> See
> http://people.mozilla.org/~jorendorff/es6-draft.html#sec-date.prototype.getyear
> step 1 and the definition of "this time value" in
> http://people.mozilla.org/~jorendorff/es6-draft.html#sec-properties-of-the-date-prototype-object
>
>
> (BTW, did you intentionally pick an Annex B Date method?)
>
> Allen
>
>
>
> --
> Cheers,
> --MarkM
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