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instanceof Object does not work with the in operatorΒ #51007
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Activity
You can just use
typeof table === 'object', which will also work for objects without prototype chain (e.g.Object.create(null)).dragomirtitian commented
on Sep 30, 2022 ContributorAuthorMartin Johns (@MartinJohns) That is the workaround I used, forgot to mention it. I still think ideally it should work.
I think
instanceofis preferable sincetypeofcan return"function"for things that are stillinstanceof ObjectThe
unknowntype gets narrowed down to{}thanks to!!table. Should we then allow a{}->object(Object?) transition through thistable instanceof Object?I don't see why not -- narrowing should generally be transitive (i.e. if
A -x-> Cis ok thenA -y-> B -x-> Cshould be as well)Should we open a new issue for this problem, or does it belong here?
export function isHTMLTable<T extends object | null>(table: T): boolean { return !!table && 'html' in table; // error! // Type 'NonNullable<T>' may represent a primitive value, which is not permitted as // the right operand of the 'in' operator.(2638) }
Joe Calzaretta (@jcalz) I'd say that this is a different problem as the issue here, at least from my PoV, was very related to
instanceof. The issue in this playground is related to generics, this one works just alright:export function isHTMLTable(table: object | null): boolean { return !!table && 'html' in table; }
Reacted by Joe Calzaretta- locked as resolved and limited conversation to collaborators
on Oct 22, 2025
Bug Report
π Search Terms
instanceof Objectinπ Version & Regression Information
β― Playground Link
Playground Link
π» Code
π Actual behavior
Error: Type '{}' may represent a primitive value, which is not permitted as the right operand of the 'in' operator
π Expected behavior
Should type check
Notes
Seems to be introduced by #50666 which correctly introduces an error if
inis used on non objects. The issue is that it does not handle testing for objects withinstanceof Object