Make optional any properties whose types that have a union with null

NullableToOptional.ts

/**
 * Inspired by this article by Rafael Bardini:
 * {@link https://rbardini.com/making-optional-properties-nullable-typescript/}
 */

type NullUnion<T> = T | null;

// type NullableProps<T> = {
//   [K in keyof T]: NullUnion<T[K]> extends T[K] ? K : never;
// }[keyof T];

// type NonNullableProps<T> = {
//   [K in keyof T]: NullUnion<T[K]> extends T[K] ? never : K;
// }[keyof T];

// type PickNullable<T> = Pick<T, NullableProps<T>>;

// type PickNonNullable<T> = Pick<T, NonNullableProps<T>>;

// type NullableToOptional<T> = Partial<PickNullable<T>> & PickNonNullable<T>;

/**
 * Converts any properties that have a union with null into optional properties.
 * 
 * @example
 * type Sample = {
 *   propA: string | null;
 *   propB: number | null;
 *   required: boolean;
 * }
 * 
 * type Test = NullableToOptional<Sample>;
 * 
 * type Expected = {
 *   propA?: string | null;
 *   propB?: number | null;
 *   required: boolean;
 * }
 * 
 * // types Test and Expected are equivalent.
 */
type NullableToOptional<T> = Partial<Pick<T, {
  [K in keyof T]: NullUnion<T[K]> extends T[K] ? K : never;
}[keyof T]>> & Pick<T, {
  [K in keyof T]: NullUnion<T[K]> extends T[K] ? never : K;
}[keyof T]>;

/*************************************/

type Test = NullableToOptional<{
  propA: string | null;
  propB: number | null;
  required: boolean;
}>;

const obj: Test = {
  required: true
};

@rbardini Thank you for the inspiring blog post. Valeu mesmo!

@joemaffei De nada! I'm glad you found it useful 🙂