TypeScript Utility Types and Recipes

type MaybeUser = { id: string } | null | undefined;
type User = NonNullable<MaybeUser>;

# Remove `null` and `undefined` from a type.

type Shape = "circle" | "square" | 42;
type StringShape = Extract<Shape, string>;

# Keep only members assignable to another type.

type Shape = "circle" | "square" | 42;
type NumericFree = Exclude<Shape, number>;

# Remove members assignable to another type.

type Fn = (id: string, active: boolean) => void;
type FnArgs = Parameters<Fn>; // [id: string, active: boolean]

# Extract a function’s parameter tuple.

class UserService {}
type UserServiceInstance = InstanceType<typeof UserService>;

# Get the instance type from a class constructor.

type Paginated<T> = {
  items: T[];
  total: number;
  page: number;
  pageSize: number;
};

# Model paginated collections with generics.

type FieldErrors<T> = Partial<Record<keyof T, string>>;

# Type validation errors by field name.

type AsyncState<T> =
  | { status: "idle" }
  | { status: "loading" }
  | { status: "success"; data: T }
  | { status: "error"; error: string };

# Represent async UI states safely.

type Events = {
  "user.created": { id: string };
  "user.deleted": { id: string };
};

# Model event payloads by event name.