TypeScript Readonly and Immutability

Readonly prevents modification after creation.

  ## Readonly property
  
  ```ts
  interface User {
    readonly id: number;
    name: string;
  }
  ```
  
  ## Readonly array
  
  ```ts
  let nums: readonly number[] = [1, 2, 3];
  // nums.push(4); ❌
  ```
  
  ## Readonly utility
  
  ```ts
  type User = { id: number; name: string };
  type ReadUser = Readonly<User>;
  ```
  
  ## Why important?
  - Prevents bugs
  - Makes intent clear
  
  ## Graph
  
  ```mermaid
  flowchart LR
    A[Mutable] --> B[Can change]
    C[Readonly] --> D[Safe state]
  ```
  
  ## Remember
  - Use readonly for safety
  - Especially for configs and state