What are Awaited and NonNullable utility types?

Awaited<T> — recursively unwraps the resolved type of a Promise (or thenable). Added in TypeScript 4.5.

type A = Awaited<Promise<string>>;
// string

type B = Awaited<Promise<Promise<number>>>;
// number (recursively unwrapped)

// Common pattern: get the type a fetch function resolves to
async function getUser(): Promise<User> { /* ... */ }

type ResolvedUser = Awaited<ReturnType<typeof getUser>>;
// User (not Promise<User>)

// Useful for typing the result of Promise.all
type MultiResult = Awaited<Promise<[User, Post[]]>>;
// [User, Post[]]

NonNullable<T> — removes null and undefined from a type:

type MaybeUser = User | null | undefined;

type DefiniteUser = NonNullable<MaybeUser>; // User

// Useful after null checks in generics
function assertDefined<T>(val: T): NonNullable<T> {
  if (val === null || val === undefined) {
    throw new Error('Value is null or undefined');
  }
  return val as NonNullable<T>;
}

// Filter nulls from an array
function compact<T>(arr: (T | null | undefined)[]): NonNullable<T>[] {
  return arr.filter((x): x is NonNullable<T> => x != null);
}