What causes the compiler to produce different results for these two mapped types?
type NonNullableObj1<O> = {[Key in keyof O] : O[Key] extends null ? never : O[Key]}
type NotNull<T> = T extends null ? never : T;
type NonNullableObj2<T> = {[P in keyof T]: NotNull<T[P]>}
type Nullable = {prop: string | null};
type ResultOf1 = NonNullableObj1<Nullable>; // { prop: string | null }
type ResultOf2 = NonNullableObj2<Nullable>; // { prop: string }
The reason for the different results between both scenarios lies in the concept of distributive conditional types.
When dealing with NotNull, the generic type T is considered as a standalone entity on one side of the condition. If a union is provided for T, each individual member of that union gets processed separately within the condition. Consequently, in this case, only string - not null - passes the check as it does not inherit from null.
In contrast, with NonNullableObj1, the situation differs because when evaluating
O[Key]</code, it's not treated as an isolated generic type due to indexed access. Therefore, the entire union of <code>string | nullstring nor null extends null, the result defaults to O[Key], which retains its original structure of string | null.
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