Mapped Types in TypeScript: Transform Every Property

Mapped Types in TypeScript allow creating new types based on existing ones by iterating over keys and modifying them.

It's like map() for types — you can iterate over each object key and set the needed value for it.

Syntax

type NewType = {
  [Key in Union]: Type;
}

• Key — variable name representing current key from Union set.
• Type — value type for each key.
• Can use keyof, modifiers (?, readonly), and utility types (Pick, Partial, etc.).

Usage Examples

Example 1: Type copy with different values

type User = {
  name: string;
  age: number;
};

// Make type where all values are boolean
type UserPermissions = {
  [K in keyof User]: boolean;
};

// => { name: boolean; age: boolean }

Example 2: All fields optional (like Partial<T>)

type Optional<T> = {
  [K in keyof T]?: T[K];
};

Usage

type User = { name: string; age: number };
type OptionalUser = Optional<User>;
// => { name?: string; age?: number }

Example 3: Make all fields readonly (like Readonly<T>)

type ReadOnly<T> = {
  readonly [K in keyof T]: T[K];
};

Example 4: Remove readonly

type Mutable<T> = {
  -readonly [K in keyof T]: T[K];
};

Where Mapped Types are Used

• When creating utility types (Partial, Pick, Readonly, Record, etc.)
• For generating dynamic objects with predefined keys
• For automatic type transformations based on other types
• For type introspection in large projects (especially with GraphQL, API, Form systems)