TypeScript Mapped Types

Mapped types let you create new types from existing ones.

    ## Basic mapped type
    
    ```ts
    type Flags<T> = {
      [K in keyof T]: boolean;
    };
    
    type User = { id: number; name: string };
    type UserFlags = Flags<User>;
    ```
    
    UserFlags becomes:
    { id: boolean; name: boolean }
    
    ## Make all readonly
    
    ```ts
    type ReadonlyType<T> = {
      readonly [K in keyof T]: T[K];
    };
    ```
    
    ## Graph
    ```mermaid
    flowchart LR
      A[keyof T] --> B[Loop keys]
      B --> C[New type]
    ```
    
    ## Remember
    - Mapped types transform properties
    - Used in many utility types