Add IsNullable and IsOptional

[jinyongp]

I'd like to add IsNullable and IsOptional types to check if they contain null and undefined, respectively.

type IsNullable<T> = Extract<T, null> extends never ? false : true
type IsOptional<T> = Extract<T, undefined> extends never ? false : true

type T1 = IsNullable<string> // false
type T2 = IsNullable<string | null> // true
type T3 = IsNullable<string | undefined> // false
type T4 = IsNullable<string | null | undefined> // true

type T5 = IsOptional<string> // false
type T6 = IsOptional<string | null> // false
type T7 = IsOptional<string | undefined> // true
type T8 = IsOptional<string | null | undefined> // true

I'm currently using these types to generate a Zod schema based on an interface. They help choose the appropriate type depending on whether a value can be null or undefined.

type Zodify<T extends Record<string, any>> = {
  [K in keyof T]-?: IsNullable<T[K]> extends true
    ? z.ZodNullable<z.ZodType<T[K]>>
    : IsOptional<T[K]> extends true
      ? z.ZodOptional<z.ZodType<T[K]>>
      : z.ZodType<T[K]>
};

interface Foo {
  bar: string;
  nullable: number | null;
  optional?: string;
  optionalNullable?: string | null;
}

const schema = z.object({
  bar: z.string(),
  nullable: z.number().nullable(),
  optional: z.string().optional(),
  optionalNullable: z.string().optional().nullable(), // should chain optional first
} satisfies Zodify<Foo>);

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