TypeScript utility types that aren't built into TypeScript

MakeRequired.d.ts

/**
 * Make all properties in T required
 */
declare type MakeRequired<T, K extends keyof T> = {
  [P in K]-?: T[P];
} & Omit<T, K>;

StrictUnion.d.ts

type UnionKeys<T> = T extends T ? keyof T : never;
type StrictUnionHelper<T, TAll> = T extends any
  ? T & Partial<Record<Exclude<UnionKeys<TAll>, keyof T>, undefined>>
  : never;
/**
 * This allows union types that are "strict" or exclusive - making all extra properties never'ed.
 *
 * @example
 * type NonStrict = { id: string } | { ns: string };
 * // OK:
 * const ok: NonStrict = { id: '123', ns: 'something' };
 * type Strict = StrictUnion<{ id: string } | { ns: string }>;
 * // ERROR:
 * const err: Strict = { id: '123', ns: 'something' };
 */
declare type StrictUnion<T> = StrictUnionHelper<T, T>;