An assert-guard to ensure item is in the enum

[winter kraken]

...

[steady grove]

onkeltem#0

Playground Link

Preview:ts function assertEnumItem<T extends object>( item: unknown | undefined, enumType: T, enumName: string ): asserts item is T | undefined { if (!Object.values(enumType).includes(item as T)) { throw new Error( `Unknown ${enumName} item: "${item}"` ) } ...

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[winter kraken]

What I don't understand is why the val is of the undefined type at the end?

There are two cases:
1)

declare const val: unknown | undefined
assertEnumItem(val, Foo, 'Foo')

This one results to typeof Foo | undefined
2)

declare const val: Foo | undefined
assertEnumItem(val, Foo, 'Foo')

This one results to just undefined

This is odd because the item first function parameter is declared as item: unknown | undefined so how it can be "sensitive" to a specific type?

[zenith canyon]

typeof Foo is not the same as Foo

for an enum Foo, Foo is one of its members, while typeof Foo refers to the entire enum object

their intersection is never

for T being typeof Foo, you want asserts item is T[keyof T] | undefined

this is a pretty good example of why typeguards are unsafe lol

your logic is right, but the typing you wrote for the logic isn't

[winter kraken]

Oh, you're right! Hahah..

Thanks @zenith canyon