> > 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 <[email protected]> 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 > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
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