What are Exclude and Extract utility types?

Both work on union types to filter members based on a condition.

Exclude<T, U> — removes from T all types assignable to U:

type T = string | number | boolean | null | undefined;

type NonNullable<T> = Exclude<T, null | undefined>;
type Primitives = Exclude<T, null | undefined>;
// string | number | boolean

type StringOrNumber = Exclude<string | number | boolean, boolean>;
// string | number

Extract<T, U> — keeps from T only types assignable to U (opposite of Exclude):

type Strings = Extract<string | number | boolean, string | boolean>;
// string | boolean

// Practical use: filter union to only the callable types
type FnOnly = Extract<string | number | (() => void), Function>;
// () => void

Combined with discriminated unions:

type Action =
  | { type: 'ADD'; payload: string }
  | { type: 'REMOVE'; id: number }
  | { type: 'CLEAR' };

// Extract only actions with a payload
type ActionWithPayload = Extract<Action, { payload: any }>;
// { type: 'ADD'; payload: string }

// Exclude the CLEAR action
type MutatingActions = Exclude<Action, { type: 'CLEAR' }>;
// { type: 'ADD'; payload: string } | { type: 'REMOVE'; id: number }