Advanced Types

TypeScript's type system is remarkably expressive. Beyond basic types and generics, it provides mapped types, conditional types, the infer keyword, and a rich set of built-in utility types. These advanced features allow you to create types that transform other types, enforce complex constraints, and eliminate boilerplate.

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Mapped Types

Mapped types create new types by transforming each property of an existing type:

// Make all properties optional
type MyPartial<T> = {
  [K in keyof T]?: T[K];
};

// Make all properties readonly
type MyReadonly<T> = {
  readonly [K in keyof T]: T[K];
};

// Make all properties nullable
type Nullable<T> = {
  [K in keyof T]: T[K] | null;
};

interface User {
  id: number;
  name: string;
  email: string;
}

type PartialUser = MyPartial<User>;
// { id?: number; name?: string; email?: string }

type ReadonlyUser = MyReadonly<User>;
// { readonly id: number; readonly name: string; readonly email: string }

Key Remapping (TypeScript 4.1+)

You can remap keys in mapped types using the as clause:

type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};