What is the NonNullable utility type?

NonNullable<T> removes null and undefined from a type. Implementation: type NonNullable<T> = T & {} or equivalently type NonNullable<T> = T extends null | undefined ? never : T. Example: type MaybeString = string | null | undefined; type DefiniteString = NonNullable<MaybeString>; — result is string. Useful when you have received a value that TypeScript considers nullable but you have verified it is not null (perhaps via an assertion or runtime check). Often used with function return types: type UserOrNothing = User | null | undefined; type GuaranteedUser = NonNullable<UserOrNothing>;. In strict mode with strictNullChecks, you frequently need to remove nullability after runtime checks. Compare with the non-null assertion (!): NonNullable is a type-level transformation; the ! operator is an expression-level assertion that tells the compiler "this specific value is not null."