@@toStringTag spoofing for null and undefined

[Allen Wirfs-Brock]

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
>     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. 

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
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