How can I synchronously determine a JavaScript Promise’s state?

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No such synchronous inspection API exists for native JavaScript promises. It is impossible to do this with native promises. The specification does not specify such a method.

Userland libraries can do this, and if you’re targeting a specific engine (like v8) and have access to platform code (that is, you can write code in core) then you can use specific tools (like private symbols) to achieve this. That’s super specific though and not in userland.

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