Great question — answering as the UphillTech team:
When you’re working with ShadowRealm , you’re essentially spinning up a completely isolated execution environment, kind of like a JS multiverse where each realm has its own version of the standard built-ins (like Object , Array , etc.). That means the Object constructor and its prototype in the ShadowRealm is not the same as the one in your main realm they may look alike, but under the hood, they’re entirely separate references. So when you do something like: objFromRealm instanceof Object // false …it’s false because you’re asking, “Is this object from the ShadowRealm an instance of my realm’s Object ?” And the answer is no it’s an instance of their Object. This is why...
Full answer by Ziqian Huang: https://stackoverflow.stackexchange.com/a/79699765 (CC BY-SA 4.0).
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