A simple example is `toJSON`. If an object defines that method, it'll get invoked automatically by JSON.stringify and it could have arbitrary side effects.
I think it's less about side effects being common when serializing, just that their fast path avoids anything that could have side effects (like toJSON).
The article touches briefly on this.
Right, I get that you could define something to have side effects. My question is why would you? What are some expected visible side effects of converting something to json?
That said, I see that they called it out as you say, now. When I first read it, I thought they watched for side effects.
I'm assuming they have an allow list on all standard types. The date types, in particular, often have a toJson that seems like it should still be used? (Or am I wrong on that, too? :D )