Is there a reliable way to distinguish between the types void and any in TypeScript?
type IsAny = void extends any ? true : false // true
type IsVoid = any extends void ? true : false // true
It seems that based on the above code snippet, it's not straightforward to determine whether a type is void or any. Are there any alternative methods to differentiate between these two types?
void and any are not mutually exclusive: a void is also considered an any.
This can be observed by combining them in a union type. If they were truly exclusive, void would not remain when only any is present:
type T = void | any // results in any
Due to this, it makes sense that your IsAny resolves to true.
On the other hand, for your IsVoid type, it evaluates to boolean, which essentially represents true | false. By substituting true and false with something else like 1 and 2, you will end up with 1 | 2.
This behavior stems from the fact that the any type could either be exactly void or something different than void. Therefore, we cannot definitively state if it extends void or not.
If you wish to determine whether a type is specifically any with no additional characteristics, or precisely void without any adjustments, you can utilize the following approach (derived from this provided solution):
type IsStrictAny<T> = 0 extends (1 & T) ? T : never;
type A = IsStrictAny<any> // results in any
type B = IsStrictAny<void> // leads to never
type IsVoid<T> = T extends void ? T : never
type IsNotStrictAny<T> = T extends IsStrictAny<T> ? never : T
type IsStrictVoid<T> = IsVoid<T> & IsNotStrictAny<T>
type C = IsStrictVoid<any> // results in never
type D = IsStrictVoid<void> // results in void
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