TypeScript Utility Types

// Deep partial
type DeepPartial<T> = T extends object
  ? { [K in keyof T]?: DeepPartial<T[K]> }
  : T;

// Non-nullable keys only
type RequiredKeys<T> = {
  [K in keyof T]-?: undefined extends T[K] ? never : K;
}[keyof T];

// Flatten union to intersection
type UnionToIntersection<U> =
  (U extends unknown ? (x: U) => void : never) extends (x: infer I) => void
    ? I
    : never;

// Pick by value type
type PickByValue<T, V> = {
  [K in keyof T as T[K] extends V ? K : never]: T[K];
};

// Readonly deep
type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K];
};