Web State Mobx

MobX State Management Patterns

Quick Guide: Use MobX for complex client state needing automatic dependency tracking, computed values, and fine-grained reactivity. Use makeAutoObservable for stores, observer from mobx-react-lite for React components, and runInAction/flow for async state updates. Never use MobX for server state -- use your data-fetching solution instead.

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST call makeAutoObservable(this) in EVERY class store constructor - or use makeObservable with explicit annotations for subclassed stores)

(You MUST wrap ALL state mutations after await in runInAction() - or use flow with generator functions instead of async/await)

(You MUST wrap EVERY React component that reads observables in observer() from mobx-react-lite)

(You MUST always dispose reactions (autorun, reaction, when) to prevent memory leaks)

Auto-detection: MobX, makeAutoObservable, makeObservable, observable, observer, mobx-react-lite, runInAction, flow, computed, autorun, reaction, useLocalObservable

When to use:

• Complex client state with computed derivations and automatic dependency tracking
• Class-based or factory-function stores with observable properties
• Fine-grained reactivity where only affected components re-render
• State that benefits from transparent reactive programming (spreadsheet-like derivations)

When NOT to use:

• Server/API data (use your data-fetching solution)
• Simple local UI state (use useState)
• Lightweight shared state without computed needs (simpler state solutions exist)
• State that should be URL-shareable (use searchParams)

Philosophy

MobX embraces a core principle: "Anything that can be derived from the application state, should be derived. Automatically." It uses transparent reactive programming where observables track dependencies at runtime and only notify exactly the computations and components that depend on changed values.

MobX uses mutable observables with automatic tracking. This means less boilerplate than immutable/reducer-based approaches but requires understanding how reactivity works -- specifically, MobX tracks property access during tracked function execution, not variable assignments.

When to Use MobX

• Complex domain models with many derived/computed values
• Applications where class-based stores provide natural organization
• Scenarios requiring fine-grained reactivity (large lists, frequent updates)
• Teams comfortable with mutable state and OOP patterns

When NOT to Use MobX

• Server state management (use your data-fetching solution)
• Simple shared UI state without derivations (lighter alternatives exist)
• Projects preferring immutable state patterns
• Simple component-local state (useState is sufficient)

Core Patterns

Pattern 1: Store Creation with makeAutoObservable

makeAutoObservable infers annotations automatically: properties become observable, getters become computed, methods become action, and generator functions become flow. It cannot be used on classes with super or that are subclassed.

class TodoStore {
  todos: Todo[] = [];
  filter: "active" | "completed" | "all" = "all";

  constructor() {
    makeAutoObservable(this); // auto-infers all annotations
  }

  get activeTodos(): Todo[] {
    return this.todos.filter((todo) => todo.status === ACTIVE_STATUS);
  }

  addTodo(title: string): void {
    this.todos.push({ id: crypto.randomUUID(), title, status: ACTIVE_STATUS });
  }
}

Use autoBind: true option to auto-bind methods for safe callback passing. Pass overrides as second argument to exclude properties (e.g., injected dependencies) from observability.

See examples/core.md for complete examples with autoBind and overrides.

Pattern 2: Store Creation with makeObservable

makeObservable requires explicit annotation of each property. Required for classes using extends (inheritance) -- makeAutoObservable throws on subclasses.

class BaseEntityStore<T extends Entity> {
  entities: T[] = [];

  constructor() {
    makeObservable(this, {
      entities: observable,
      entityCount: computed,
      addEntity: action,
    });
  }

  get entityCount(): number {
    return this.entities.length;
  }
}

See examples/core.md for base/subclass examples.

Pattern 3: Factory Function Stores

Factory functions with makeAutoObservable avoid this and new complexity, compose easily, and can hide private members via closures.

function createTimerStore(): TimerStore {
  return makeAutoObservable({
    secondsPassed: INITIAL_SECONDS,
    get minutesPassed(): number {
      return Math.floor(this.secondsPassed / SECONDS_PER_MINUTE);
    },
    tick(): void {
      this.secondsPassed++;
    },
  });
}

See examples/core.md for typed factory examples.

Pattern 4: React Integration with observer

The observer HOC from mobx-react-lite makes React components reactive. It automatically tracks which observables are read during render and re-renders only when those specific values change. observer auto-applies React.memo.

// observer tracks observables read during render
const TodoList = observer(function TodoList() {
  return (
    <ul>
      {todoStore.filteredTodos.map((todo) => (
        <TodoItem key={todo.id} todo={todo} /> {/* Pass objects, NOT primitives */}
      ))}
    </ul>
  );
});

Critical: Pass observable objects to child components, not destructured primitives. Extracting primitives before the observer boundary breaks fine-grained tracking.

See examples/core.md for observer, Context-based DI, and anti-patterns.

Pattern 5: useLocalObservable for Local Component State

useLocalObservable creates a local observable store scoped to a component. Use for complex local state with computed values -- not for simple boolean toggles (useState suffices).

See examples/core.md for multi-step form example.

Pattern 6: Computed Values

Computed values are derivations that automatically cache and recalculate when their dependencies change. Should be pure (no side effects). Use computed.struct for structural comparison when output shape matters more than reference.

get subtotal(): number {
  return this.items.reduce((sum, item) => sum + item.price * item.quantity, 0);
}
get total(): number {
  return this.subtotal + this.tax + this.shippingCost; // chains computed values
}

See examples/advanced.md for chained computeds and computed.struct.

Pattern 7: Actions and runInAction

Actions are the only place you should modify observable state. They batch mutations into transactions, so reactions only fire after the outermost action completes.

See examples/advanced.md for batching and enforceActions examples.

Pattern 8: Async Patterns (flow and runInAction)

Code after await runs in a new tick and is NOT part of the original action. Two solutions:

// Option A: runInAction after await
async fetchUsers(): Promise<void> {
  this.isLoading = true; // OK: before await
  const users = await this.api.getUsers();
  runInAction(() => { this.users = users; this.isLoading = false; }); // MUST wrap
}

// Option B (recommended): flow with generators - no wrapping needed
*fetchUsers() {
  this.isLoading = true;
  this.users = yield this.api.getUsers(); // auto-wrapped in action context
  this.isLoading = false;
}

flow returns a cancellable promise with .cancel(). makeAutoObservable auto-infers generators as flow. Use flowResult() to cast the generator return for TypeScript type inference; import CancellablePromise from "mobx" for the return type.

See examples/advanced.md for complete examples.

Pattern 9: Reactions (autorun, reaction, when)

Reactions bridge reactive MobX state to imperative side effects. ALL reactions return a disposer -- you MUST call it to prevent memory leaks.

• autorun: Runs immediately and re-runs whenever any read observable changes
• reaction: Data function + effect function -- effect only runs when data function return value changes (not on init)
• when: Runs once when predicate becomes true, then auto-disposes. Without an effect function, returns a Promise.

Reactions only track observables read synchronously -- not in setTimeout, promises, or after await.

See examples/advanced.md for all three reaction types with React cleanup patterns.

Pattern 10: Root Store Pattern

The root store pattern organizes multiple domain and UI stores into a single coordinator that enables cross-store communication via shared reference.

class RootStore {
  userStore: UserStore;
  todoStore: TodoStore;

  constructor(transportLayer: TransportLayer) {
    this.userStore = new UserStore(this, transportLayer);
    this.todoStore = new TodoStore(this, transportLayer);
  }
}

Provide the root store via React Context (dependency injection, NOT state management).

See examples/architecture.md for full root store with domain stores, UI store, provider, and convenience hooks.

Pattern 11: TypeScript Integration

MobX has first-class TypeScript support. Use makeAutoObservable<Store, "privateField"> to annotate private fields. Class stores get type inference automatically; factory functions should return typed interfaces.

See examples/architecture.md for typed stores and private field examples.

Pattern 12: Performance Optimization

MobX provides fine-grained reactivity, but component structure matters. Use many small observer components and dereference observables as late as possible (pass objects, not extracted primitives).

See examples/architecture.md for list rendering, toJS, and configure() examples.

Detailed Resources:

• examples/core.md - Store creation, observer, useLocalObservable
• examples/advanced.md - Computed values, actions, async, reactions
• examples/architecture.md - Root store, TypeScript, performance

Decision Framework

makeAutoObservable vs makeObservable

Does the store class use inheritance (extends)?
|-- YES --> makeObservable (explicit annotations required)
|-- NO --> Is the store subclassed by other stores?
    |-- YES --> makeObservable (makeAutoObservable forbids subclassing)
    |-- NO --> makeAutoObservable (less boilerplate, auto-inference)

Async Pattern: flow vs runInAction

Is the async operation complex with multiple yields?
|-- YES --> flow (generator function, cancellable, cleaner)
|-- NO --> Is cancellation needed?
    |-- YES --> flow (returns promise with .cancel())
    |-- NO --> runInAction (simpler for single await)

Reaction Type Selection

Need to run effect immediately and on every change?
|-- YES --> autorun
|-- NO --> Need to run effect only when specific data changes?
    |-- YES --> reaction (data function + effect function)
    |-- NO --> Need to run effect once when condition is true?
        |-- YES --> when
        |-- NO --> Reconsider if you need a reaction at all

When to Use MobX vs Alternatives

Is it server data (from API)?
|-- YES --> Not MobX's scope. Use your data-fetching solution.
|-- NO --> Is it simple local UI state (one component)?
    |-- YES --> useState
    |-- NO --> Does it need computed/derived values?
        |-- YES --> Do you prefer OOP / class-based stores?
        |   |-- YES --> MobX
        |   |-- NO --> Consider your state management solution's derived selectors
        |-- NO --> Is it lightweight shared state?
            |-- YES --> A simpler state solution may suffice
            |-- NO --> MobX (fine-grained reactivity scales well)

RED FLAGS

High Priority Issues:

• Mutating observables outside actions -- breaks MobX enforceActions, causes unpredictable state updates
• Missing observer wrapper on components reading observables -- component will not re-render when state changes (most common MobX bug)
• Not disposing reactions -- autorun, reaction, when all return disposers that MUST be called to prevent memory leaks
• State mutation after await without runInAction -- code after await is NOT in the original action, will fail with enforceActions

Medium Priority Issues:

• Using mobx-react instead of mobx-react-lite (heavier, includes class component support you likely do not need)
• Destructuring primitives from observables outside tracked functions (breaks reactivity tracking)
• Using makeAutoObservable on subclassed stores (will throw -- use makeObservable instead)
• Creating side effects in computed values (computeds must be pure derivations)
• Overusing reactions where computed values would suffice

Common Mistakes:

• Reading observables in setTimeout/setInterval callbacks without proper tracking
• Passing extracted primitive values instead of observable objects to child components (breaks fine-grained tracking)
• Forgetting autoBind: true when passing store methods as callbacks (leads to lost this context)
• Using rest destructuring (...store) on observables (touches all properties, makes component overly reactive)
• Not using toJS() when passing observable data to non-MobX-aware libraries

Gotchas and Edge Cases:

• observer auto-applies React.memo -- never wrap an observer component in memo again (redundant)
• Computed values suspend when not observed -- accessing them outside reactions causes recalculation every time (use keepAlive option if needed, but watch for memory leaks)
• autorun tracks only synchronous reads -- observables read in async callbacks, promises, or after await are NOT tracked
• reaction does NOT run on initialization (unlike autorun) -- use fireImmediately: true option if needed
• Generator functions are automatically inferred as flow by makeAutoObservable -- do not also wrap them in flow(). However, some transpiler configurations cannot detect generators; if flow does not work as expected, specify flow explicitly in overrides
• action.bound and autoBind: true are NOT the same as arrow function class fields -- arrow functions cannot be overridden in subclasses. flow.bound works the same way for generator methods
• MobX tracks property access ("arrows"), not values -- reassigning a variable that held an observable reference does NOT trigger reactions
• Reactions accept a signal: AbortSignal option as an alternative to manual disposer calls -- useful when tying reaction lifetime to an AbortController

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST call makeAutoObservable(this) in EVERY class store constructor - or use makeObservable with explicit annotations for subclassed stores)

(You MUST wrap ALL state mutations after await in runInAction() - or use flow with generator functions instead of async/await)

(You MUST wrap EVERY React component that reads observables in observer() from mobx-react-lite)

(You MUST always dispose reactions (autorun, reaction, when) to prevent memory leaks)

Failure to follow these rules will break MobX reactivity, cause memory leaks, or produce stale data.