null2undefined.ts

export default function null2undefined<T, K extends keyof T>(
  o: T,
  name: K,
): NullableAsUndefined<T>[K] {
  const val = o[name];
  return val || undefined;   // Error on this line --
                             // Type 'T[K] | undefined' is not assignable to
                             // type 'T[K] extends infer U | null ? U | undefined : T[K]'.
                             // Type 'undefined' is not assignable to
                             // type 'T[K] extends infer U | null ? U | undefined : T[K]'.
}

type NullableAsUndefined<T> = {
  [K in keyof T]: T[K] extends (infer U | null) ? U | undefined : T[K]
};

// Similar concept:
function defaultingGeneric<T>(x: T): NonNullable<T> | undefined {
  return x || undefined;  // Error on this line --
                          // Type 'T | undefined' is not assignable to
                          // type 'NonNullable<T> | undefined'.
                          // Type 'T' is not assignable to
                          // type 'NonNullable<T> | undefined'.
                          // Type 'T' is not assignable to
                          // type 'NonNullable<T>'.
}

// No error on this one:
function defaultingString(x: string | null): NonNullable<typeof x> | undefined {
  return x || undefined;
}