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