`instanceof Object` does not work with the `in` operator

[dragomirtitian]

Bug Report

🔎 Search Terms

instanceof Object in

🕗 Version & Regression Information

• This changed between versions 4.9.0-dev.20220919 and 4.9.0-dev.20220920

⏯ Playground Link

Playground Link

💻 Code

export function isHTMLTable(table: unknown): boolean {
    return !!table && table instanceof Object && 'html' in table;
}

🙁 Actual behavior

Error: Type '{}' may represent a primitive value, which is not permitted as the right operand of the 'in' operator

🙂 Expected behavior

Should type check

Notes

Seems to be introduced by #50666 which correctly introduces an error if in is used on non objects. The issue is that it does not handle testing for objects with instanceof Object

[MartinJohns]

You can just use typeof table === 'object', which will also work for objects without prototype chain (e.g. Object.create(null)).

[dragomirtitian]

@MartinJohns That is the workaround I used, forgot to mention it. I still think ideally it should work.

[RyanCavanaugh]

I think instanceof is preferable since typeof can return "function" for things that are still instanceof Object

[Andarist]

The unknown type gets narrowed down to {} thanks to !!table. Should we then allow a {} -> object (Object?) transition through this table instanceof Object?

[RyanCavanaugh]

I don't see why not -- narrowing should generally be transitive (i.e. if A -x-> C is ok then A -y-> B -x-> C should be as well)

[jcalz]

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)
}

Playground link

[Andarist]

@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;
}