This makes me wonder: for objects with [[NumberData]] and [[StringData]], why do they call `valueOf`/`toString`/@@toPrimitive to coerce them instead of simply accessing the boxed data directly? It seems like a pointless waste to check.
----- Isiah Meadows cont...@isiahmeadows.com www.isiahmeadows.com On Sun, Aug 5, 2018 at 12:30 AM, Michael Theriot <michael.lee.ther...@gmail.com> wrote: >> Regarding cases like Reflect.construct(Number, [], String), the reason >> this throws is because the ToNumber algorithm calls ToPrimitive if its >> operand is an object. This in turn will call String.prototype.valueOf on the >> object which does not have [[StringData]]. There’s nothing funny going on, >> it’s just a weird effect in aggregate. You would just need to implement all >> the steps here — internal ops like ToNumber, ToPrimitive, etc. It’s not that >> it "considers type", but rather that these algorithms will call methods by >> specific names on objects they receive. String.prototype and >> Object.prototype both implement "valueOf", but only the former will fail the >> slot check. > > > Thanks for demystifying this. I never realized it was just ducktyping > .valueOf on the object. > > ```js > Number.prototype.valueOf = () => 0; > > JSON.stringify(1); // 1 > JSON.stringify(new Number(1)); // 0 > ``` > > I guess what confused me is that it would detect a [[NumberData]] internal > slot, but instead of unboxing to that value it tries to ducktype call > .valueOf on the object. So the presence of [[NumberData]] changes the > behavior even if it does not actually use this value. > > On Sat, Aug 4, 2018 at 10:14 PM Darien Valentine <valentin...@gmail.com> > wrote: >> >> Is this a question about how/if one could replicate its behavior in theory >> from ES code, or are you suggesting a change to the existing behavior for >> these exotic cases? >> >> Assuming the former: >> >> The relevant steps are >> [here](https://tc39.github.io/ecma262/#sec-serializejsonproperty). The >> `instanceof` a value isn’t significant in this algorithm, just its slots (if >> it is an object) or its primitive type. So one can handle number and string >> by leveraging branded checks as you’ve shown — nothing more is needed than >> branded methods and typeof to manage that one. >> >> However it is ultimately not possible to replicate because there is no >> possible brand test for [[BooleanData]]. >> _______________________________________________ >> es-discuss mailing list >> es-discuss@mozilla.org >> https://mail.mozilla.org/listinfo/es-discuss > > > _______________________________________________ > es-discuss mailing list > es-discuss@mozilla.org > https://mail.mozilla.org/listinfo/es-discuss > _______________________________________________ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
