Mutable Global State (ARCH3187)

Key typescript.quality.mutable-global-state · Severity warning · Category quality

Module-level mutable shared state creates hidden dependencies and test-isolation problems, and leads to unpredictable behavior under concurrent access. State should be encapsulated or kept immutable.

Incorrect

export let currentUser: User | null = null;

export function login(u: User) {
  currentUser = u;
}

Correct

export class Session {
  private user: User | null = null;
  setUser(u: User) { this.user = u; }
  getUser() { return this.user; }
}

Fix

Encapsulate shared state inside a class/service and inject its instance as a dependency.