Implementing an identical JSON.stringify

Michael Theriot michael.lee.theriot at
Sun Aug 5 04:30:05 UTC 2018

> 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.

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 <valentinium at>

> 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](
> 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]].
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