Skip to content

typescript.global_mutable_state

Correctness High

Detects module-level mutable state.

Why It Matters

Global mutable state causes subtle bugs:

Request leakage - In serverless, state persists between invocations
Race conditions - Concurrent requests modify shared state
Testing pain - State leaks between tests
Debugging difficulty - State changes from anywhere

Example

// ❌ Before
let cache: Map<string, User> = new Map();
let requestCount = 0;

export function getUser(id: string) {
    requestCount++;
    if (!cache.has(id)) {
        cache.set(id, fetchUser(id));
    }
    return cache.get(id);
}

// ✅ After
// Encapsulate in class
class UserService {
    private cache = new Map<string, User>();

    getUser(id: string) {
        if (!this.cache.has(id)) {
            this.cache.set(id, this.fetchUser(id));
        }
        return this.cache.get(id);
    }
}

// Or use dependency injection
export function createUserService(cache: Cache) {
    return {
        getUser(id: string) {
            return cache.get(id) ?? fetchUser(id);
        }
    };
}

What Unfault Detects

let at module level with mutable values
Module-level arrays, objects, Maps, Sets
Global variables that get reassigned

Auto-Fix

Unfault suggests encapsulating state or using const.