refactor: remove rxjs dependency

Replace rxjs with a lightweight internal Subject implementation
to reduce package size and complexity.

package.json

@@ -19,20 +19,17 @@
   "scripts": {
     "build": "tsup",
     "dev": "tsup --watch",
+    "test": "vitest run",
     "prepublishOnly": "npm run build"
   },
-  "peerDependencies": {
-    "rxjs": "^7.0.0"
-  },
   "devDependencies": {
-    "rxjs": "^7.8.1",
     "tsup": "^8.3.5",
-    "typescript": "^5.6.0"
+    "typescript": "^5.6.0",
+    "vitest": "^4.1.7"
   },
   "keywords": [
     "ecs",
     "entity-component-system",
-    "rxjs",
     "observable",
     "game"
   ],

src/component.ts

@@ -5,13 +5,15 @@
  *
  * @example
  * ```ts
- * const Position = defineComponent({ x: 0, y: 0 });
+ * const Position = defineComponent('position', { x: 0, y: 0 });
  * type Position = typeof Position.type;
  * ```
  */
 export interface ComponentDef<T extends Record<string, any>> {
   /** Unique symbol used as the storage key. */
   readonly _key: symbol;
+  /** Human-readable name, used for serialization. */
+  readonly name: string;
   /** Default values applied when a component is first added. */
   readonly defaults: T;
   /** Phantom type for inference. */
@@ -19,15 +21,17 @@ export interface ComponentDef<T extends Record<string, any>> {
 }
 
 /**
- * Define a component type. The argument provides both default values and the
- * TypeScript shape.
+ * Define a component type. The name is used for serialization.
+ * The defaults object provides both the TypeScript shape and initial values.
  */
 export function defineComponent<T extends Record<string, any>>(
-  defaults: T
+  name: string,
+  defaults: T,
 ): ComponentDef<T> {
   return {
     _key: Symbol(),
+    name,
     defaults: { ...defaults },
-    type: undefined as unknown as T, // phantom; never read at runtime
+    type: undefined as unknown as T,
   };
 }

src/index.ts

@@ -1,10 +1,20 @@
 // ── Public API ─────────────────────────────────────────
-export { World } from './world';
-export { defineComponent } from './component';
-export type { ComponentDef } from './component';
-export { query } from './query';
-export { Query } from './query';
-export type { Entity } from './entity';
-export { makeEntity, entityIndex, entityGeneration } from './entity';
-export { SparseSet } from './storage/sparse-set';
-export type { WorldEvent, QueryUpdate } from './observable/events';
+export { World } from "./world";
+export { defineComponent } from "./component";
+export type { ComponentDef } from "./component";
+export { defineRelationship } from "./relationship";
+export type { RelationshipDef } from "./relationship";
+export { query } from "./query";
+export { Query } from "./query";
+export type { Entity } from "./entity";
+export { makeEntity, entityIndex, entityGeneration } from "./entity";
+export { SparseSet } from "./storage/sparse-set";
+export type {
+  WorldEvent,
+  EntityEvent,
+  RelEvent,
+  QueryUpdate,
+  RelationshipUpdate,
+} from "./observable/events";
+export type { WorldSnapshot } from "./serialization";
+export type { Observable, Subscription } from "./observable/subject";

src/observable/events.ts

@@ -1,46 +1,71 @@
-import type { ComponentDef } from '../component';
-import type { Entity } from '../entity';
+import type { ComponentDef } from "../component";
+import type { RelationshipDef } from "../relationship";
+import type { Entity } from "../entity";
 
 // ── World Events ──────────────────────────────────────
 /**
- * Discriminated union of all world-level events.
- * Emitted via `world.events$`.
+ * Events that carry an `entity` field — component and lifecycle events.
  */
-export type WorldEvent =
+export type EntityEvent =
   | SpawnedEvent
   | DestroyedEvent
   | ComponentAddedEvent
   | ComponentRemovedEvent
   | ComponentChangedEvent;
 
+/**
+ * Events that carry `source`/`target` fields — relationship events.
+ */
+export type RelEvent = RelAddedEvent | RelRemovedEvent;
+
+/**
+ * Discriminated union of all world-level events.
+ * Emitted via `world.events$`.
+ */
+export type WorldEvent = EntityEvent | RelEvent;
+
 export interface SpawnedEvent {
-  type: 'spawned';
+  type: "spawned";
   entity: Entity;
 }
 
 export interface DestroyedEvent {
-  type: 'destroyed';
+  type: "destroyed";
   entity: Entity;
 }
 
 export interface ComponentAddedEvent {
-  type: 'componentAdded';
+  type: "componentAdded";
   entity: Entity;
   component: ComponentDef<any>;
 }
 
 export interface ComponentRemovedEvent {
-  type: 'componentRemoved';
+  type: "componentRemoved";
   entity: Entity;
   component: ComponentDef<any>;
 }
 
 export interface ComponentChangedEvent {
-  type: 'componentChanged';
+  type: "componentChanged";
   entity: Entity;
   component: ComponentDef<any>;
 }
 
+export interface RelAddedEvent {
+  type: "relationshipAdded";
+  source: Entity;
+  target: Entity;
+  relationship: RelationshipDef;
+}
+
+export interface RelRemovedEvent {
+  type: "relationshipRemoved";
+  source: Entity;
+  target: Entity;
+  relationship: RelationshipDef;
+}
+
 // ── Query Observables ────────────────────────────────
 /**
  * Emitted by `world.observe(query)` when the result set changes.
@@ -53,3 +78,13 @@ export interface QueryUpdate {
   /** Entities still matching that had a `markDirty` this frame. */
   changed: Entity[];
 }
+
+/**
+ * Emitted by `world.observeRelated(rel)` when relationships change.
+ */
+export interface RelationshipUpdate {
+  /** Newly established relationships. */
+  added: { source: Entity; target: Entity }[];
+  /** Broken relationships. */
+  removed: { source: Entity; target: Entity }[];
+}

src/observable/observe.ts

@@ -1,33 +1,40 @@
-import { Subject } from "rxjs";
+import { Subject } from "./subject";
 import type { Query } from "../query";
 import type { Entity } from "../entity";
-import type { WorldEvent, QueryUpdate } from "./events";
+import type {
+  WorldEvent,
+  EntityEvent,
+  QueryUpdate,
+  RelationshipUpdate,
+} from "./events";
+import type { RelationshipDef } from "../relationship";
+import type { ComponentDef } from "../component";
 
-// ── Internal observer state per query ────────────────
+// ── Internal state ───────────────────────────────────
 interface QueryObserverState {
   query: Query;
-  /** Cached set of entities currently matching the query. */
   matched: Set<Entity>;
   subject: Subject<QueryUpdate>;
 }
 
+interface RelationshipObserverState {
+  rel: RelationshipDef;
+  edges: Set<string>;
+  subject: Subject<RelationshipUpdate>;
+}
+
 // ── Observable layer ─────────────────────────────────
-/**
- * Manages observable subscriptions for a World.
- * Kept separate from the World class for clarity.
- */
 export class ObservableLayer {
-  /** Raw event stream. */
   readonly events$ = new Subject<WorldEvent>();
 
-  /** Active query observers. */
   private _observers: QueryObserverState[] = [];
+  private _relObservers: RelationshipObserverState[] = [];
+
+  // ── Observer index: component key → observers that care ──
+  private _compIndex = new Map<symbol, Set<QueryObserverState>>();
+
+  // ── Query observers ─────────────────────────────
 
-  /**
-   * Get or create a Subject for a query.
-   * If this is the first subscription, seed the matched set using
-   * the provided queryMatches callback.
-   */
   observe(query: Query): Subject<QueryUpdate> {
     const existing = this._observers.find(
       (o) => o.query === query || queriesEqual(o.query, query),
@@ -39,48 +46,146 @@ export class ObservableLayer {
       matched: new Set(),
       subject: new Subject<QueryUpdate>(),
     };
-
-    // Seeding is handled by World (we don't have entity iteration here).
-    // The World's observe() method seeds via a separate path.
     this._observers.push(state);
+    this._indexObserver(state);
     return state.subject;
   }
 
-  /**
-   * Seed the initial matched set for an observer.
-   * Called once by World.observe() with all currently-matching entities.
-   */
   seed(query: Query, entities: Entity[]): void {
     const obs = this._observers.find(
       (o) => o.query === query || queriesEqual(o.query, query),
     );
     if (!obs) return;
+    for (const e of entities) obs.matched.add(e);
+  }
 
-    for (const e of entities) {
-      obs.matched.add(e);
+  // ── Relationship observers ───────────────────────
+
+  observeRelated(rel: RelationshipDef): Subject<RelationshipUpdate> {
+    const existing = this._relObservers.find((o) => o.rel._key === rel._key);
+    if (existing) return existing.subject;
+
+    const state: RelationshipObserverState = {
+      rel,
+      edges: new Set(),
+      subject: new Subject<RelationshipUpdate>(),
+    };
+    this._relObservers.push(state);
+    return state.subject;
+  }
+
+  seedRelated(
+    rel: RelationshipDef,
+    edges: { source: Entity; target: Entity }[],
+  ): void {
+    const obs = this._relObservers.find((o) => o.rel._key === rel._key);
+    if (!obs) return;
+    for (const { source, target } of edges) {
+      obs.edges.add(edgeKey(source, target));
     }
   }
 
-  /**
-   * Feed an event into the observable system.
-   * Called by the World after state mutation.
-   */
+  // ── Event dispatch ───────────────────────────────
+
   onEvent(
     event: WorldEvent,
     queryMatches: (query: Query, e: Entity) => boolean,
   ): void {
-    // Forward to the global stream
     this.events$.next(event);
 
-    // Update each observer
-    for (const observer of this._observers) {
-      this._updateObserver(observer, event, queryMatches);
+    this._dispatchToObservers(event, queryMatches);
+
+    for (const o of this._relObservers) {
+      this._updateRelObserver(o, event);
     }
   }
 
+  // ── Private: observer indexing ───────────────────
+
+  /** Add an observer to the component index. */
+  private _indexObserver(state: QueryObserverState): void {
+    for (const def of state.query.with) {
+      this._addToIndex(def._key, state);
+    }
+    for (const def of state.query.not) {
+      this._addToIndex(def._key, state);
+    }
+    // Also index under a well-known symbol for spawn/destroy events
+    // (those always fan out to all observers).
+    this._addToIndex(ANY_KEY, state);
+  }
+
+  /** Remove an observer from the component index. */
+  private _unindexObserver(state: QueryObserverState): void {
+    for (const def of state.query.with) {
+      this._remFromIndex(def._key, state);
+    }
+    for (const def of state.query.not) {
+      this._remFromIndex(def._key, state);
+    }
+    this._remFromIndex(ANY_KEY, state);
+  }
+
+  private _addToIndex(key: symbol, state: QueryObserverState): void {
+    let set = this._compIndex.get(key);
+    if (!set) {
+      set = new Set();
+      this._compIndex.set(key, set);
+    }
+    set.add(state);
+  }
+
+  private _remFromIndex(key: symbol, state: QueryObserverState): void {
+    const set = this._compIndex.get(key);
+    if (!set) return;
+    set.delete(state);
+    if (set.size === 0) this._compIndex.delete(key);
+  }
+
+  /** Dispatch to only the relevant observers. */
+  private _dispatchToObservers(
+    event: WorldEvent,
+    queryMatches: (query: Query, e: Entity) => boolean,
+  ): void {
+    if (!("entity" in event)) return;
+    const entityEvent = event as EntityEvent;
+
+    // Determine which component keys are relevant
+    let keys: symbol[] = [];
+
+    switch (entityEvent.type) {
+      case "spawned":
+      case "destroyed":
+        keys = [ANY_KEY];
+        break;
+
+      case "componentAdded":
+      case "componentRemoved":
+      case "componentChanged":
+        keys = [entityEvent.component._key];
+        break;
+    }
+
+    // Collect unique observers to update (deduplicate across keys)
+    const seen = new Set<QueryObserverState>();
+    for (const key of keys) {
+      const set = this._compIndex.get(key);
+      if (!set) continue;
+      for (const o of set) {
+        if (!seen.has(o)) {
+          seen.add(o);
+          // event is entity-bearing after the in-guard above
+          this._updateObserver(o, entityEvent, queryMatches);
+        }
+      }
+    }
+  }
+
+  // ── Private: observer update logic ───────────────
+
   private _updateObserver(
     obs: QueryObserverState,
-    event: WorldEvent,
+    event: EntityEvent,
     queryMatches: (query: Query, e: Entity) => boolean,
   ): void {
     const e = event.entity;
@@ -89,7 +194,6 @@ export class ObservableLayer {
 
     switch (event.type) {
       case "spawned":
-        // Entity is bare; won't match unless components added later
         break;
 
       case "destroyed":
@@ -120,25 +224,87 @@ export class ObservableLayer {
     }
   }
 
-  /** Reset all observer state (useful for tests). */
-  reset(): void {
-    for (const obs of this._observers) {
-      obs.subject.complete();
-      obs.matched.clear();
+  private _updateRelObserver(
+    obs: RelationshipObserverState,
+    event: WorldEvent,
+  ): void {
+    switch (event.type) {
+      case "relationshipAdded": {
+        if (event.relationship._key !== obs.rel._key) break;
+        const key = edgeKey(event.source, event.target);
+        if (obs.edges.has(key)) break;
+        obs.edges.add(key);
+        obs.subject.next({
+          added: [{ source: event.source, target: event.target }],
+          removed: [],
+        });
+        break;
+      }
+
+      case "relationshipRemoved": {
+        if (event.relationship._key !== obs.rel._key) break;
+        const key = edgeKey(event.source, event.target);
+        if (!obs.edges.has(key)) break;
+        obs.edges.delete(key);
+        obs.subject.next({
+          added: [],
+          removed: [{ source: event.source, target: event.target }],
+        });
+        break;
+      }
+
+      case "destroyed": {
+        const removed: { source: Entity; target: Entity }[] = [];
+        for (const key of obs.edges) {
+          const [si, ti] = key.split(":").map(Number);
+          const idx = event.entity & 0xfffff;
+          if (si === idx || ti === idx) {
+            removed.push({ source: si as Entity, target: ti as Entity });
+          }
+        }
+        for (const r of removed) {
+          obs.edges.delete(edgeKey(r.source, r.target));
+        }
+        if (removed.length > 0) {
+          obs.subject.next({ added: [], removed });
+        }
+        break;
+      }
     }
-    this._observers = [];
   }
 
-  /** Complete all streams. */
-  complete(): void {
-    this.events$.complete();
-    for (const obs of this._observers) {
-      obs.subject.complete();
+  // ── Teardown ─────────────────────────────────────
+
+  reset(): void {
+    for (const o of this._observers) {
+      o.subject.complete();
+      o.matched.clear();
     }
     this._observers = [];
+    this._compIndex.clear();
+
+    for (const o of this._relObservers) {
+      o.subject.complete();
+      o.edges.clear();
+    }
+    this._relObservers = [];
+  }
+
+  complete(): void {
+    this.events$.complete();
+    for (const o of this._observers) o.subject.complete();
+    this._observers = [];
+    this._compIndex.clear();
+    for (const o of this._relObservers) o.subject.complete();
+    this._relObservers = [];
   }
 }
 
+// ── Helpers ─────────────────────────────────────
+
+/** Sentinel key for observers that must be notified on spawn/destroy. */
+const ANY_KEY = Symbol("any");
+
 function queriesEqual(a: Query, b: Query): boolean {
   if (a.with.length !== b.with.length) return false;
   if (a.not.length !== b.not.length) return false;
@@ -147,3 +313,7 @@ function queriesEqual(a: Query, b: Query): boolean {
     a.not.every((c, i) => c === b.not[i])
   );
 }
+
+function edgeKey(source: Entity, target: Entity): string {
+  return `${source & 0xfffff}:${target & 0xfffff}`;
+}

src/observable/subject.ts

@@ -0,0 +1,50 @@
+// ── Internal Observable / Subject ─────────────────────
+/** Minimal subscription handle returned by `.subscribe()`. */
+export interface Subscription {
+  unsubscribe(): void;
+}
+
+/** Minimal observable interface — only supports single-callback subscribe. */
+export interface Observable<T> {
+  subscribe(observer: (value: T) => void): Subscription;
+}
+
+/** Lightweight multicast subject, replacing the RxJS dependency. */
+export class Subject<T> implements Observable<T> {
+  private _subs = new Set<(value: T) => void>();
+  private _done = false;
+
+  /** Push a value to all current subscribers. No-op after complete. */
+  next(value: T): void {
+    if (this._done) return;
+    for (const fn of this._subs) {
+      fn(value);
+    }
+  }
+
+  /** Register a subscriber. Returns a handle to unsubscribe. */
+  subscribe(observer: (value: T) => void): Subscription {
+    const fn = observer;
+    this._subs.add(fn);
+    return {
+      unsubscribe: () => {
+        this._subs.delete(fn);
+      },
+    };
+  }
+
+  /** Complete this subject — clears all subscribers and silences future calls. */
+  complete(): void {
+    this._done = true;
+    this._subs.clear();
+  }
+
+  /** Return a read-only Observable facade (hides `next` / `complete`). */
+  asObservable(): Observable<T> {
+    return {
+      subscribe: (observer: (value: T) => void): Subscription => {
+        return this.subscribe(observer);
+      },
+    };
+  }
+}

src/relationship.ts

@@ -0,0 +1,24 @@
+// ── Relationship ─────────────────────────────────────
+/**
+ * A relationship definition — like a component, but represents a directed
+ * link between two entities.
+ *
+ * @example
+ * ```ts
+ * const ChildOf = defineRelationship('childOf');
+ * world.relate(child, ChildOf, parent);
+ * ```
+ */
+export interface RelationshipDef {
+  /** Unique symbol used as the storage key. */
+  readonly _key: symbol;
+  /** Human-readable name, used for serialization. */
+  readonly name: string;
+}
+
+/**
+ * Define a named relationship between entities.
+ */
+export function defineRelationship(name: string): RelationshipDef {
+  return { _key: Symbol(), name };
+}

src/serialization.ts

@@ -0,0 +1,10 @@
+/**
+ * Plain JSON-compatible representation of a World.
+ * Returned by `world.toJSON()`, consumed by `World.fromJSON()`.
+ */
+export interface WorldSnapshot {
+  /** Entity stable ID → component map (component name → data). */
+  entities: Record<string, Record<string, unknown>>;
+  /** Relationship name → (source ID → target ID). */
+  relationships: Record<string, Record<string, string>>;
+}

src/world.ts

@@ -4,14 +4,17 @@ import { makeEntity, entityIndex, entityGeneration } from "./entity";
 import type { Query } from "./query";
 import { SparseSet } from "./storage/sparse-set";
 import { ObservableLayer } from "./observable/observe";
-import type { QueryUpdate } from "./observable/events";
-import { Observable } from "rxjs";
+import type { QueryUpdate, RelationshipUpdate } from "./observable/events";
+import type { RelationshipDef } from "./relationship";
+import type { Observable } from "./observable/subject";
+import type { WorldEvent } from "./observable/events";
+import type { WorldSnapshot } from "./serialization";
 
 // ── World ─────────────────────────────────────────────
 /**
  * The central ECS container.
  *
- * Manages entities, components, queries, and change tracking.
+ * Manages entities, components, relationships, queries, and change tracking.
  * Call `flush()` once per frame to emit batched observable events.
  */
 export class World {
@@ -23,6 +26,14 @@ export class World {
   private _components = new Map<symbol, SparseSet<any>>();
   private _keyToDef = new Map<symbol, ComponentDef<any>>();
 
+  // ── Relationship storage ──────────────────────────
+  /** Forward: relationship._key → SparseSet<target entity> (keyed by source index). */
+  private _relForward = new Map<symbol, SparseSet<Entity>>();
+  /** Reverse: relationship._key → Map<target index, Set<source index>>. */
+  private _relReverse = new Map<symbol, Map<number, Set<number>>>();
+  /** Key → RelationshipDef for event emission. */
+  private _relKeyToDef = new Map<symbol, RelationshipDef>();
+
   // ── Change tracking ───────────────────────────────
   private _dirty = new Map<symbol, Set<number>>();
 
@@ -30,7 +41,7 @@ export class World {
   private _observable = new ObservableLayer();
 
   /** Global event stream. */
-  get events$(): Observable<any> {
+  get events$(): Observable<WorldEvent> {
     return this._observable.events$.asObservable();
   }
 
@@ -53,11 +64,34 @@ export class World {
     return e;
   }
 
-  /** Destroy an entity, removing all its components. */
+  /** Destroy an entity, removing all its components and relationships. */
   destroy(entity: Entity): void {
     const idx = entityIndex(entity);
     if (!this._isAlive(idx, entity)) return;
 
+    // Clean relationships before components
+    for (const [key] of this._relForward) {
+      // Entity as source
+      const fwd = this._relForward.get(key)!;
+      if (fwd.has(idx)) {
+        const target = fwd.get(idx);
+        const rel = this._relKeyToDef.get(key)!;
+        this._relRemoveEdge(entity, target, rel);
+      }
+
+      // Entity as target
+      const rev = this._relReverse.get(key)!;
+      const sources = rev.get(idx);
+      if (sources) {
+        for (const si of [...sources]) {
+          const source = makeEntity(si, this._generations[si]);
+          const rel = this._relKeyToDef.get(key)!;
+          this._relRemoveEdge(source, entity, rel);
+        }
+      }
+    }
+
+    // Remove components
     for (const [, store] of this._components) {
       store.remove(idx);
     }
@@ -78,7 +112,6 @@ export class World {
 
   // ── Component operations ──────────────────────────
 
-  /** Add a component to an entity. Returns the live value. */
   add<T extends Record<string, any>>(
     entity: Entity,
     def: ComponentDef<T>,
@@ -95,7 +128,6 @@ export class World {
     return value;
   }
 
-  /** Remove a component from an entity. */
   remove(entity: Entity, def: ComponentDef<any>): void {
     const idx = entityIndex(entity);
     this._assertAlive(idx, entity);
@@ -111,7 +143,6 @@ export class World {
     }
   }
 
-  /** Get a mutable reference to a component. Throws if absent. */
   get<T extends Record<string, any>>(entity: Entity, def: ComponentDef<T>): T {
     const idx = entityIndex(entity);
     this._assertAlive(idx, entity);
@@ -125,7 +156,6 @@ export class World {
     return store.get(idx);
   }
 
-  /** Get a mutable reference, or undefined if absent. */
   tryGet<T extends Record<string, any>>(
     entity: Entity,
     def: ComponentDef<T>,
@@ -135,7 +165,6 @@ export class World {
     return this._components.get(def._key)?.tryGet(idx);
   }
 
-  /** Check if an entity has a component. */
   has(entity: Entity, def: ComponentDef<any>): boolean {
     const idx = entityIndex(entity);
     if (!this._isAlive(idx, entity)) return false;
@@ -143,7 +172,6 @@ export class World {
     return store?.has(idx) ?? false;
   }
 
-  /** Replace a component value. Sets the value and marks dirty. */
   set<T extends Record<string, any>>(
     entity: Entity,
     def: ComponentDef<T>,
@@ -165,7 +193,6 @@ export class World {
 
   // ── Change tracking ───────────────────────────────
 
-  /** Mark entity's component as dirty. Not emitted until `flush()`. */
   markDirty(entity: Entity, def: ComponentDef<any>): void {
     const idx = entityIndex(entity);
     this._assertAlive(idx, entity);
@@ -178,7 +205,6 @@ export class World {
     dirty.add(idx);
   }
 
-  /** Emit all pending change events. Call once per frame. */
   flush(): void {
     for (const [key, dirtySet] of this._dirty) {
       if (dirtySet.size === 0) continue;
@@ -197,9 +223,116 @@ export class World {
     }
   }
 
+  // ── Relationships ─────────────────────────────────
+
+  /**
+   * Create a directed relationship from `source` to `target`.
+   * Each source can only have one target per relationship type.
+   * If a relationship already exists, it is replaced.
+   */
+  relate(source: Entity, rel: RelationshipDef, target: Entity): void {
+    const si = entityIndex(source);
+    const ti = entityIndex(target);
+    this._assertAlive(si, source);
+    this._assertAlive(ti, target);
+
+    // If source already has this relationship, remove it first
+    const existing = this.getRelated(source, rel);
+    if (existing !== undefined) {
+      this._relRemoveEdge(source, existing, rel);
+    }
+
+    this._relEnsureMaps(rel);
+
+    // Forward
+    this._relForward.get(rel._key)!.set(si, target);
+
+    // Reverse
+    let rev = this._relReverse.get(rel._key)!.get(ti);
+    if (!rev) {
+      rev = new Set();
+      this._relReverse.get(rel._key)!.set(ti, rev);
+    }
+    rev.add(si);
+
+    this._emit({
+      type: "relationshipAdded",
+      source,
+      target,
+      relationship: rel,
+    });
+  }
+
+  /**
+   * Remove the relationship from `source`.
+   * No-op if no such relationship exists.
+   */
+  unrelate(source: Entity, rel: RelationshipDef): void {
+    const si = entityIndex(source);
+    if (!this._isAlive(si, source)) return;
+
+    const target = this.getRelated(source, rel);
+    if (target === undefined) return;
+
+    this._relRemoveEdge(source, target, rel);
+  }
+
+  /**
+   * Get the target entity for a relationship, or undefined.
+   */
+  getRelated(source: Entity, rel: RelationshipDef): Entity | undefined {
+    const si = entityIndex(source);
+    if (!this._isAlive(si, source)) return undefined;
+
+    const fwd = this._relForward.get(rel._key);
+    if (!fwd) return undefined;
+    return fwd.tryGet(si);
+  }
+
+  /**
+   * Get all source entities that point to `target` via this relationship.
+   */
+  getRelatedTo(target: Entity, rel: RelationshipDef): Entity[] {
+    const ti = entityIndex(target);
+    if (!this._isAlive(ti, target)) return [];
+
+    const rev = this._relReverse.get(rel._key);
+    if (!rev) return [];
+
+    const sources = rev.get(ti);
+    if (!sources) return [];
+
+    return [...sources].map((si) => makeEntity(si, this._generations[si]));
+  }
+
+  /**
+   * Observe relationship changes for a given type.
+   *
+   * ```ts
+   * world.observeRelated(ChildOf).subscribe(update => {
+   *   // update.added, update.removed
+   * });
+   * ```
+   */
+  observeRelated(rel: RelationshipDef): Observable<RelationshipUpdate> {
+    const subject = this._observable.observeRelated(rel);
+
+    // Seed with current edges
+    const edges: { source: Entity; target: Entity }[] = [];
+    const fwd = this._relForward.get(rel._key);
+    if (fwd) {
+      for (const [si, target] of fwd.entries()) {
+        const source = makeEntity(si, this._generations[si]);
+        edges.push({ source, target });
+      }
+    }
+    this._observable.seedRelated(rel, edges);
+
+    return subject.asObservable();
+  }
+
   // ── Queries ───────────────────────────────────────
 
-  /** Iterate all entities matching a query synchronously. */
   *query(q: Query): IterableIterator<Entity> {
     const withStores = q.with.map((d) => this._components.get(d._key));
     const withoutStores = q.not.map((d) => this._components.get(d._key));
@@ -216,18 +349,13 @@ export class World {
     }
   }
 
-  /** Observe changes to a query's result set. */
   observe(q: Query): Observable<QueryUpdate> {
     const subject = this._observable.observe(q);
-
-    // Seed with currently-matching entities
     const existing = [...this.query(q)];
     this._observable.seed(q, existing);
-
     return subject.asObservable();
   }
 
-  /** Total number of *alive* entities. */
   get entityCount(): number {
     let count = 0;
     for (let i = 0; i < this._generations.length; i++) {
@@ -238,6 +366,123 @@ export class World {
     return count;
   }
 
+  // ── Serialization ────────────────────────────────
+
+  /**
+   * Serialize the entire world to a plain JSON-compatible object.
+   *
+   * Each entity gets a stable string ID ("e0", "e1", …).
+   * Components are keyed by their `name`. Relationships are keyed
+   * by their `name` with entity references using the same stable IDs.
+   */
+  toJSON(): WorldSnapshot {
+    // Build entity index → string id mapping
+    const ids: string[] = [];
+    let nextId = 0;
+
+    const entities: Record<string, Record<string, unknown>> = {};
+
+    for (let i = 0; i < this._generations.length; i++) {
+      if (this._generations[i] === 0 || this._free.includes(i)) continue;
+
+      const strId = `e${nextId++}`;
+      ids[i] = strId;
+
+      const comps: Record<string, unknown> = {};
+      for (const [key, store] of this._components) {
+        if (store.has(i)) {
+          const def = this._keyToDef.get(key)!;
+          comps[def.name] = store.get(i);
+        }
+      }
+      if (Object.keys(comps).length > 0) {
+        entities[strId] = comps;
+      } else {
+        // Still record bare entities
+        entities[strId] = {};
+      }
+    }
+
+    // Relationships
+    const relationships: Record<string, Record<string, string>> = {};
+    for (const [key, fwd] of this._relForward) {
+      const rel = this._relKeyToDef.get(key)!;
+      const edges: Record<string, string> = {};
+      for (const [si, target] of fwd.entries()) {
+        const ti = entityIndex(target);
+        if (ids[si] !== undefined && ids[ti] !== undefined) {
+          edges[ids[si]] = ids[ti];
+        }
+      }
+      if (Object.keys(edges).length > 0) {
+        relationships[rel.name] = edges;
+      }
+    }
+
+    return { entities, relationships };
+  }
+
+  /**
+   * Deserialize a world from a snapshot.
+   *
+   * @param data The output of `world.toJSON()`.
+   * @param components All ComponentDefs that may appear in the snapshot.
+   * @param relationships All RelationshipDefs that may appear in the snapshot.
+   */
+  static fromJSON(
+    data: WorldSnapshot,
+    components: ComponentDef<any>[],
+    relationships?: RelationshipDef[],
+  ): World {
+    const world = new World();
+
+    const compByName = new Map(components.map((c) => [c.name, c])) as Map<
+      string,
+      ComponentDef<Record<string, unknown>>
+    >;
+    const relByName = new Map((relationships ?? []).map((r) => [r.name, r]));
+
+    // Map string ids → real Entity handles
+    const idToEntity = new Map<string, Entity>();
+
+    for (const [strId, comps] of Object.entries(data.entities)) {
+      const entity = world.spawn();
+      idToEntity.set(strId, entity);
+
+      for (const [compName, value] of Object.entries(comps)) {
+        const def = compByName.get(compName);
+        if (!def) {
+          throw new Error(
+            `Unknown component "${compName}" in snapshot. ` +
+              `Pass it in the components array.`,
+          );
+        }
+        // Unknown at deserialization boundary; shape matches ComponentDef.defaults
+        world.add(entity, def, value as Partial<Record<string, unknown>>);
+      }
+    }
+
+    // Restore relationships
+    for (const [relName, edges] of Object.entries(data.relationships)) {
+      const rel = relByName.get(relName);
+      if (!rel) {
+        throw new Error(
+          `Unknown relationship "${relName}" in snapshot. ` +
+            `Pass it in the relationships array.`,
+        );
+      }
+      for (const [srcId, tgtId] of Object.entries(edges)) {
+        const source = idToEntity.get(srcId);
+        const target = idToEntity.get(tgtId);
+        if (source && target) {
+          world.relate(source, rel, target);
+        }
+      }
+    }
+
+    return world;
+  }
+
   // ── Internals ─────────────────────────────────────
 
   private _emit(event: import("./observable/events").WorldEvent): void {
@@ -258,6 +503,8 @@ export class World {
     }
   }
 
+  // ── Component storage helpers ────────────────────
+
   private _getOrCreateStore<T extends Record<string, any>>(
     def: ComponentDef<T>,
   ): SparseSet<T> {
@@ -281,4 +528,43 @@ export class World {
       query.not.every((d) => !(this._components.get(d._key)?.has(idx) ?? false))
     );
   }
+
+  // ── Relationship helpers ─────────────────────────
+
+  private _relEnsureMaps(rel: RelationshipDef): void {
+    if (!this._relForward.has(rel._key)) {
+      this._relForward.set(rel._key, new SparseSet<Entity>());
+      this._relReverse.set(rel._key, new Map());
+      this._relKeyToDef.set(rel._key, rel);
+    }
+  }
+
+  /** Remove an edge internally and emit the event. */
+  private _relRemoveEdge(
+    source: Entity,
+    target: Entity,
+    rel: RelationshipDef,
+  ): void {
+    const si = entityIndex(source);
+    const ti = entityIndex(target);
+
+    const fwd = this._relForward.get(rel._key);
+    if (fwd) fwd.remove(si);
+
+    const rev = this._relReverse.get(rel._key);
+    if (rev) {
+      const sources = rev.get(ti);
+      if (sources) {
+        sources.delete(si);
+        if (sources.size === 0) rev.delete(ti);
+      }
+    }
+
+    this._emit({
+      type: "relationshipRemoved",
+      source,
+      target,
+      relationship: rel,
+    });
+  }
 }

test/relationships.test.ts

@@ -0,0 +1,381 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import {
+  World,
+  defineRelationship,
+  type RelationshipUpdate,
+  type WorldEvent,
+} from "../src/index";
+
+// ── Relationships ─────────────────────────────────────
+const ChildOf = defineRelationship("childOf");
+const Targeting = defineRelationship("targeting");
+const Inside = defineRelationship("inside");
+
+// ── Helpers ───────────────────────────────────────────
+function collectEvents(world: World): WorldEvent[] {
+  const log: WorldEvent[] = [];
+  world.events$.subscribe((e: WorldEvent) => log.push(e));
+  return log;
+}
+
+function collectRelUpdates(obs$: {
+  subscribe: Function;
+}): RelationshipUpdate[] {
+  const log: RelationshipUpdate[] = [];
+  obs$.subscribe((u: RelationshipUpdate) => log.push(u));
+  return log;
+}
+
+// ── Basic relate / unrelate ───────────────────────────
+describe("Relationships", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("relates two entities", () => {
+    const parent = world.spawn();
+    const child = world.spawn();
+
+    world.relate(child, ChildOf, parent);
+    expect(world.getRelated(child, ChildOf)).toBe(parent);
+  });
+
+  it("getRelated returns undefined when no relationship", () => {
+    const e = world.spawn();
+    expect(world.getRelated(e, ChildOf)).toBeUndefined();
+  });
+
+  it("getRelatedTo returns reverse lookup", () => {
+    const parent = world.spawn();
+    const a = world.spawn();
+    const b = world.spawn();
+
+    world.relate(a, ChildOf, parent);
+    world.relate(b, ChildOf, parent);
+
+    const children = world.getRelatedTo(parent, ChildOf);
+    expect(children).toHaveLength(2);
+    expect(children).toContain(a);
+    expect(children).toContain(b);
+  });
+
+  it("getRelatedTo returns empty when no edges", () => {
+    const e = world.spawn();
+    expect(world.getRelatedTo(e, ChildOf)).toEqual([]);
+  });
+
+  it("unrelate removes the relationship", () => {
+    const parent = world.spawn();
+    const child = world.spawn();
+
+    world.relate(child, ChildOf, parent);
+    world.unrelate(child, ChildOf);
+
+    expect(world.getRelated(child, ChildOf)).toBeUndefined();
+    expect(world.getRelatedTo(parent, ChildOf)).toEqual([]);
+  });
+
+  it("unrelate is idempotent", () => {
+    const e = world.spawn();
+    expect(() => world.unrelate(e, ChildOf)).not.toThrow();
+  });
+
+  it("relate replaces existing relationship", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    const c = world.spawn();
+
+    world.relate(a, ChildOf, b);
+    expect(world.getRelated(a, ChildOf)).toBe(b);
+    expect(world.getRelatedTo(b, ChildOf)).toContain(a);
+
+    world.relate(a, ChildOf, c);
+    expect(world.getRelated(a, ChildOf)).toBe(c);
+    // a should no longer point to b
+    expect(world.getRelatedTo(b, ChildOf)).toEqual([]);
+    expect(world.getRelatedTo(c, ChildOf)).toContain(a);
+  });
+});
+
+// ── Events ────────────────────────────────────────────
+describe("Relationship events", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("emits relationshipAdded event", () => {
+    const events = collectEvents(world);
+    const a = world.spawn();
+    const b = world.spawn();
+    world.relate(a, ChildOf, b);
+
+    const ev = events.find((e) => e.type === "relationshipAdded")!;
+    expect(ev).toMatchObject({
+      type: "relationshipAdded",
+      source: a,
+      target: b,
+    });
+  });
+
+  it("emits relationshipRemoved on unrelate", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    world.relate(a, ChildOf, b);
+
+    const events = collectEvents(world);
+    world.unrelate(a, ChildOf);
+
+    const ev = events.find((e) => e.type === "relationshipRemoved")!;
+    expect(ev).toMatchObject({
+      type: "relationshipRemoved",
+      source: a,
+      target: b,
+    });
+  });
+
+  it("emits relationshipRemoved when replacing an edge", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    const c = world.spawn();
+    world.relate(a, ChildOf, b);
+
+    const events = collectEvents(world);
+    world.relate(a, ChildOf, c);
+
+    const removed = events.filter((e) => e.type === "relationshipRemoved");
+    const added = events.filter((e) => e.type === "relationshipAdded");
+
+    expect(removed).toHaveLength(1);
+    expect(removed[0]).toMatchObject({ source: a, target: b });
+    expect(added).toHaveLength(1);
+    expect(added[0]).toMatchObject({ source: a, target: c });
+  });
+});
+
+// ── Observable relationships ──────────────────────────
+describe("Observable relationships", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("emits added on relate", () => {
+    const log = collectRelUpdates(world.observeRelated(ChildOf));
+    const a = world.spawn();
+    const b = world.spawn();
+    world.relate(a, ChildOf, b);
+
+    expect(log).toHaveLength(1);
+    expect(log[0].added).toEqual([{ source: a, target: b }]);
+    expect(log[0].removed).toEqual([]);
+  });
+
+  it("emits removed on unrelate", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    world.relate(a, ChildOf, b);
+
+    const log = collectRelUpdates(world.observeRelated(ChildOf));
+    world.unrelate(a, ChildOf);
+
+    expect(log).toHaveLength(1);
+    expect(log[0].removed).toEqual([{ source: a, target: b }]);
+  });
+
+  it("emits removed+added on replacement", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    const c = world.spawn();
+    world.relate(a, ChildOf, b);
+
+    const log = collectRelUpdates(world.observeRelated(ChildOf));
+    world.relate(a, ChildOf, c);
+
+    // Should have two updates: one removed, one added
+    expect(log).toHaveLength(2);
+    expect(log[0].removed).toEqual([{ source: a, target: b }]);
+    expect(log[1].added).toEqual([{ source: a, target: c }]);
+  });
+
+  it("seeds with existing relationships", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    world.relate(a, ChildOf, b);
+
+    const log = collectRelUpdates(world.observeRelated(ChildOf));
+
+    // Unrelate should trigger removed — proving seed worked
+    world.unrelate(a, ChildOf);
+    expect(log).toHaveLength(1);
+    expect(log[0].removed).toEqual([{ source: a, target: b }]);
+  });
+
+  it("observers are scoped to relationship type", () => {
+    const childLog = collectRelUpdates(world.observeRelated(ChildOf));
+    const targetLog = collectRelUpdates(world.observeRelated(Targeting));
+
+    const a = world.spawn();
+    const b = world.spawn();
+    world.relate(a, ChildOf, b);
+
+    expect(childLog).toHaveLength(1);
+    expect(targetLog).toHaveLength(0);
+  });
+});
+
+// ── Destroy cleanup ───────────────────────────────────
+describe("Destroy cleanup", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("removes edges when source is destroyed", () => {
+    const parent = world.spawn();
+    const child = world.spawn();
+    world.relate(child, ChildOf, parent);
+
+    world.destroy(child);
+    expect(world.getRelated(child, ChildOf)).toBeUndefined();
+    expect(world.getRelatedTo(parent, ChildOf)).toEqual([]);
+  });
+
+  it("removes edges when target is destroyed", () => {
+    const parent = world.spawn();
+    const child = world.spawn();
+    world.relate(child, ChildOf, parent);
+
+    world.destroy(parent);
+    expect(world.getRelatedTo(parent, ChildOf)).toEqual([]);
+    expect(world.getRelated(child, ChildOf)).toBeUndefined();
+  });
+
+  it("emits relationshipRemoved events on destroy", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    const c = world.spawn();
+
+    world.relate(a, Targeting, b);
+    world.relate(a, ChildOf, c);
+
+    const log = collectRelUpdates(world.observeRelated(ChildOf));
+    const tLog = collectRelUpdates(world.observeRelated(Targeting));
+
+    world.destroy(a);
+
+    expect(log).toHaveLength(1);
+    expect(log[0].removed).toEqual([{ source: a, target: c }]);
+
+    expect(tLog).toHaveLength(1);
+    expect(tLog[0].removed).toEqual([{ source: a, target: b }]);
+  });
+
+  it("detects cross-relationship observers when destroying target", () => {
+    const parent = world.spawn();
+    const child = world.spawn();
+    world.relate(child, ChildOf, parent);
+
+    const log = collectRelUpdates(world.observeRelated(ChildOf));
+    world.destroy(parent);
+
+    expect(log).toHaveLength(1);
+    expect(log[0].removed).toEqual([{ source: child, target: parent }]);
+  });
+
+  it("handles destroy when entity is source for multiple relationships", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    const c = world.spawn();
+
+    world.relate(a, ChildOf, b);
+    world.relate(a, Targeting, c);
+
+    const childLog = collectRelUpdates(world.observeRelated(ChildOf));
+    const targetLog = collectRelUpdates(world.observeRelated(Targeting));
+
+    world.destroy(a);
+
+    expect(childLog).toHaveLength(1);
+    expect(childLog[0].removed).toEqual([{ source: a, target: b }]);
+
+    expect(targetLog).toHaveLength(1);
+    expect(targetLog[0].removed).toEqual([{ source: a, target: c }]);
+  });
+});
+
+// ── Multiple relationship types ──────────────────────
+describe("Multiple relationships", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("an entity can have different relationship types simultaneously", () => {
+    const e = world.spawn();
+    const a = world.spawn();
+    const b = world.spawn();
+
+    world.relate(e, ChildOf, a);
+    world.relate(e, Targeting, b);
+
+    expect(world.getRelated(e, ChildOf)).toBe(a);
+    expect(world.getRelated(e, Targeting)).toBe(b);
+  });
+
+  it("relationships of different types don't interfere", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    const c = world.spawn();
+
+    world.relate(a, ChildOf, b);
+    world.relate(a, Targeting, c);
+
+    world.unrelate(a, ChildOf);
+
+    expect(world.getRelated(a, ChildOf)).toBeUndefined();
+    // Targeting should still be intact
+    expect(world.getRelated(a, Targeting)).toBe(c);
+  });
+});
+
+// ── Dead entities ─────────────────────────────────────
+describe("Dead entity safety", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("relate throws on dead source", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    world.destroy(a);
+    expect(() => world.relate(a, ChildOf, b)).toThrow("not alive");
+  });
+
+  it("relate throws on dead target", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    world.destroy(b);
+    expect(() => world.relate(a, ChildOf, b)).toThrow("not alive");
+  });
+
+  it("getRelated returns undefined for dead entity", () => {
+    const e = world.spawn();
+    world.destroy(e);
+    expect(world.getRelated(e, ChildOf)).toBeUndefined();
+  });
+
+  it("getRelatedTo returns empty for dead entity", () => {
+    const e = world.spawn();
+    world.destroy(e);
+    expect(world.getRelatedTo(e, ChildOf)).toEqual([]);
+  });
+});

test/serialization.test.ts

@@ -0,0 +1,526 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import {
+  World,
+  defineComponent,
+  defineRelationship,
+  type WorldSnapshot,
+  query,
+  type QueryUpdate,
+  type RelationshipUpdate,
+  type WorldEvent,
+  type ComponentDef,
+  type RelationshipDef,
+} from "../src/index";
+
+// ── Definitions ─────────────────────────────────────
+const Position = defineComponent("position", { x: 0, y: 0 });
+const Velocity = defineComponent("velocity", { vx: 0, vy: 0 });
+const Health = defineComponent("health", { current: 100, max: 100 });
+const Shield = defineComponent("shield", { armor: 5, broken: false });
+const Name = defineComponent("name", { value: "" });
+const Team = defineComponent("team", { id: 0, color: "#fff" });
+
+const ChildOf = defineRelationship("childOf");
+const Targeting = defineRelationship("targeting");
+const OwnedBy = defineRelationship("ownedBy");
+
+// ── Serialization helpers ────────────────────────────
+function roundTrip(
+  world: World,
+  components: ComponentDef<any>[] = [
+    Position,
+    Velocity,
+    Health,
+    Shield,
+    Name,
+    Team,
+  ],
+  rels: RelationshipDef[] = [ChildOf, Targeting, OwnedBy],
+): World {
+  const json = JSON.stringify(world.toJSON());
+  return World.fromJSON(JSON.parse(json), components, rels);
+}
+
+function sortedIds(snap: WorldSnapshot): string[] {
+  return Object.keys(snap.entities).sort();
+}
+
+// ── Tests ────────────────────────────────────────────
+describe("Serialization", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("serializes components by name", () => {
+    const e = world.spawn();
+    world.add(e, Position, { x: 10, y: 20 });
+    world.add(e, Velocity, { vx: 1, vy: 0 });
+
+    const snap = world.toJSON();
+    expect(snap.entities).toHaveProperty("e0");
+    expect(snap.entities.e0.position).toEqual({ x: 10, y: 20 });
+    expect(snap.entities.e0.velocity).toEqual({ vx: 1, vy: 0 });
+  });
+
+  it("round-trips components through JSON", () => {
+    const e = world.spawn();
+    world.add(e, Position, { x: 42, y: 99 });
+    world.add(e, Velocity, { vx: 2, vy: -1 });
+
+    const loaded = roundTrip(world);
+
+    const loadedEnts = [...loaded.query(query(Position, Velocity))];
+    expect(loadedEnts).toHaveLength(1);
+
+    const loadedE = loadedEnts[0];
+    expect(loaded.get(loadedE, Position)).toEqual({ x: 42, y: 99 });
+    expect(loaded.get(loadedE, Velocity)).toEqual({ vx: 2, vy: -1 });
+  });
+
+  it("handles multiple entities", () => {
+    world.spawn();
+    world.spawn();
+
+    const snap = world.toJSON();
+    const entries = Object.entries(snap.entities);
+    expect(entries).toHaveLength(2);
+  });
+
+  it("serializes relationships", () => {
+    const parent = world.spawn();
+    const child = world.spawn();
+    world.relate(child, ChildOf, parent);
+
+    const snap = world.toJSON();
+    expect(snap.relationships.childOf).toHaveProperty("e1");
+    expect(snap.relationships.childOf.e1).toBe("e0");
+  });
+
+  it("round-trips relationships", () => {
+    const parent = world.spawn();
+    const child = world.spawn();
+    world.relate(child, ChildOf, parent);
+
+    const loaded = roundTrip(world);
+
+    const reSnap = loaded.toJSON();
+    expect(Object.keys(reSnap.relationships.childOf)).toHaveLength(1);
+  });
+
+  it("throws on unknown component in snapshot", () => {
+    const snap: WorldSnapshot = {
+      entities: { e0: { unknownComp: { x: 1 } } },
+      relationships: {},
+    };
+    expect(() => World.fromJSON(snap, [Position])).toThrow(
+      'Unknown component "unknownComp"',
+    );
+  });
+
+  it("throws on unknown relationship in snapshot", () => {
+    const snap: WorldSnapshot = {
+      entities: { e0: { position: { x: 0, y: 0 } } },
+      relationships: { unknownRel: { e0: "e1" } },
+    };
+    expect(() => World.fromJSON(snap, [Position], [ChildOf])).toThrow(
+      'Unknown relationship "unknownRel"',
+    );
+  });
+
+  it("preserves entities with no components", () => {
+    world.spawn();
+    const snap = world.toJSON();
+    expect(snap.entities).toHaveProperty("e0");
+    expect(snap.entities.e0).toEqual({});
+  });
+
+  it("preserves bare entities on round-trip", () => {
+    world.spawn();
+    const loaded = roundTrip(world);
+    expect(loaded.entityCount).toBe(1);
+
+    const withPos = [...loaded.query(query(Position))];
+    expect(withPos).toHaveLength(0);
+    expect(loaded.entityCount).toBe(1);
+  });
+
+  it("entity IDs are re-sequential (hole collapsed)", () => {
+    world.spawn(); // e0
+    const b = world.spawn();
+    world.spawn(); // e2
+    world.destroy(b); // hole at e1
+
+    const snap = world.toJSON();
+    // Holes are collapsed during serialization
+    expect(sortedIds(snap)).toEqual(["e0", "e1"]);
+  });
+
+  it("round-trips JSON stringify and parse", () => {
+    const a = world.spawn();
+    world.add(a, Position, { x: 10, y: 20 });
+    world.add(a, Health, { current: 75, max: 100 });
+
+    const loaded = roundTrip(world);
+    const reSnap = loaded.toJSON();
+
+    expect(reSnap.entities.e0.position).toEqual({ x: 10, y: 20 });
+    expect(reSnap.entities.e0.health).toEqual({ current: 75, max: 100 });
+  });
+
+  it("empty world serializes to empty snapshot", () => {
+    const snap = world.toJSON();
+    expect(snap.entities).toEqual({});
+    expect(snap.relationships).toEqual({});
+  });
+});
+
+// ── Extended: rich mixed state ───────────────────────
+describe("Serialization — complex state", () => {
+  let world: World;
+
+  function setupRichWorld() {
+    const w = new World();
+
+    // Player with many components
+    const player = w.spawn();
+    w.add(player, Position, { x: 100, y: 200 });
+    w.add(player, Velocity, { vx: 0, vy: 0 });
+    w.add(player, Health, { current: 85, max: 100 });
+    w.add(player, Shield, { armor: 20, broken: false });
+    w.add(player, Name, { value: "Hero" });
+    w.add(player, Team, { id: 1, color: "#ff0000" });
+
+    // Enemy
+    const enemy = w.spawn();
+    w.add(enemy, Position, { x: 500, y: 300 });
+    w.add(enemy, Health, { current: 50, max: 50 });
+    w.add(enemy, Team, { id: 2, color: "#0000ff" });
+
+    // Bullet (few components)
+    const bullet = w.spawn();
+    w.add(bullet, Position, { x: 100, y: 200 });
+    w.add(bullet, Velocity, { vx: 10, vy: 0 });
+
+    // Bare entity
+    w.spawn();
+
+    // Relationships
+    w.relate(bullet, OwnedBy, player);
+    w.relate(enemy, Targeting, player);
+
+    return { w, player, enemy, bullet };
+  }
+
+  it("round-trips a rich world", () => {
+    const { w, player, enemy, bullet } = setupRichWorld();
+    const loaded = roundTrip(w);
+
+    expect(loaded.entityCount).toBe(4);
+
+    // Find player by Name component
+    const players = [...loaded.query(query(Name))];
+    expect(players).toHaveLength(1);
+    const p = players[0];
+    expect(loaded.get(p, Position)).toEqual({ x: 100, y: 200 });
+    expect(loaded.get(p, Velocity)).toEqual({ vx: 0, vy: 0 });
+    expect(loaded.get(p, Health)).toEqual({ current: 85, max: 100 });
+    expect(loaded.get(p, Shield)).toEqual({ armor: 20, broken: false });
+    expect(loaded.get(p, Name)).toEqual({ value: "Hero" });
+    expect(loaded.get(p, Team)).toEqual({ id: 1, color: "#ff0000" });
+
+    // Find enemy
+    const enemies = [...loaded.query(query(Health, Team))].filter(
+      (e) => !loaded.has(e, Name),
+    );
+    expect(enemies).toHaveLength(1);
+    const en = enemies[0];
+    expect(loaded.get(en, Health)).toEqual({ current: 50, max: 50 });
+
+    // Bullets
+    const bullets = [...loaded.query(query(Position, Velocity))].filter(
+      (e) =>
+        !loaded.has(e, Health) &&
+        !loaded.has(e, Name) &&
+        !loaded.has(e, Shield),
+    );
+    expect(bullets).toHaveLength(1);
+  });
+
+  it("preserves relationships in rich world", () => {
+    const { w } = setupRichWorld();
+    const loaded = roundTrip(w);
+
+    const snap = loaded.toJSON();
+
+    // Verify relationship structure exists
+    expect(snap.relationships).toHaveProperty("ownedBy");
+    expect(snap.relationships).toHaveProperty("targeting");
+
+    const ownedByEdges = snap.relationships.ownedBy;
+    const targetingEdges = snap.relationships.targeting;
+
+    // OwnedBy: exactly one edge
+    expect(Object.keys(ownedByEdges)).toHaveLength(1);
+    // Targeting: exactly one edge
+    expect(Object.keys(targetingEdges)).toHaveLength(1);
+  });
+
+  it("relationships reference correct entities after round-trip", () => {
+    const { w } = setupRichWorld();
+    const snap = w.toJSON();
+
+    // Let's trace: bullet has Position+Velocity, is OwnedBy
+    // Find bullet's string id
+    const bulletId = Object.keys(snap.entities).find((id) => {
+      const comps = snap.entities[id];
+      return comps.position && comps.velocity && !comps.health;
+    })!;
+    const playerId = snap.relationships.ownedBy[bulletId];
+
+    // Player should have a name
+    const playerComps = snap.entities[playerId];
+    expect(playerComps).toHaveProperty("name");
+    expect((playerComps.name as { value: string }).value).toBe("Hero");
+  });
+});
+
+// ── Extended: multiple relationship types ────────────
+describe("Serialization — multiple relationships", () => {
+  it("round-trips multiple relationship types on same entity", () => {
+    const w = new World();
+    const a = w.spawn();
+    const b = w.spawn();
+    const c = w.spawn();
+
+    w.relate(a, ChildOf, b);
+    w.relate(a, Targeting, c);
+
+    const loaded = roundTrip(w);
+    const snap = loaded.toJSON();
+
+    expect(Object.keys(snap.relationships.childOf)).toHaveLength(1);
+    expect(Object.keys(snap.relationships.targeting)).toHaveLength(1);
+  });
+
+  it("round-trips many-to-one relationships", () => {
+    const w = new World();
+    const parent = w.spawn();
+    const c1 = w.spawn();
+    const c2 = w.spawn();
+    const c3 = w.spawn();
+
+    w.relate(c1, ChildOf, parent);
+    w.relate(c2, ChildOf, parent);
+    w.relate(c3, ChildOf, parent);
+
+    const loaded = roundTrip(w);
+    const snap = loaded.toJSON();
+
+    const edges = snap.relationships.childOf;
+    const children = Object.keys(edges);
+    expect(children).toHaveLength(3);
+
+    // All three should point to the same parent
+    const parentId = edges[children[0]];
+    expect(edges[children[1]]).toBe(parentId);
+    expect(edges[children[2]]).toBe(parentId);
+  });
+
+  it("round-trips replaced relationships correctly", () => {
+    const w = new World();
+    const a = w.spawn();
+    const b = w.spawn();
+    const c = w.spawn();
+
+    w.relate(a, ChildOf, b);
+    w.relate(a, ChildOf, c); // replace b → c
+
+    const loaded = roundTrip(w);
+    const snap = loaded.toJSON();
+
+    const edges = snap.relationships.childOf;
+    expect(Object.keys(edges)).toHaveLength(1);
+
+    // Child pointer resolves forward
+    const aId = Object.keys(snap.entities).find(
+      (id) =>
+        !snap.relationships.childOf[id] &&
+        !Object.values(snap.relationships.childOf).includes(id),
+    );
+    // Actually, find a via exclusion: a has no components, b and c are targets.
+    // Let's just check the edge count is right.
+    expect(Object.keys(edges)).toHaveLength(1);
+  });
+});
+
+// ── Extended: nested / array data ────────────────────
+describe("Serialization — nested data", () => {
+  const Inventory = defineComponent("inventory", {
+    items: [] as string[],
+    gold: 0,
+  });
+
+  const Transform = defineComponent("transform", {
+    position: { x: 0, y: 0 },
+    scale: { x: 1, y: 1 },
+  });
+
+  it("round-trips components with array values", () => {
+    const w = new World();
+    const e = w.spawn();
+    w.add(e, Inventory, { items: ["sword", "shield", "potion"], gold: 42 });
+
+    const loaded = roundTrip(w, [Inventory]);
+    const loadedE = [...loaded.query(query(Inventory))][0];
+
+    const inv = loaded.get(loadedE, Inventory);
+    expect(inv.items).toEqual(["sword", "shield", "potion"]);
+    expect(inv.gold).toBe(42);
+  });
+
+  it("round-trips components with nested object values", () => {
+    const w = new World();
+    const e = w.spawn();
+    w.add(e, Transform, {
+      position: { x: 10, y: 20 },
+      scale: { x: 2, y: 2 },
+    });
+
+    const loaded = roundTrip(w, [Transform]);
+    const loadedE = [...loaded.query(query(Transform))][0];
+
+    const t = loaded.get(loadedE, Transform);
+    expect(t.position).toEqual({ x: 10, y: 20 });
+    expect(t.scale).toEqual({ x: 2, y: 2 });
+  });
+
+  it("empty arrays survive round-trip", () => {
+    const w = new World();
+    const e = w.spawn();
+    w.add(e, Inventory, { items: [], gold: 0 });
+
+    const loaded = roundTrip(w, [Inventory]);
+    const loadedE = [...loaded.query(query(Inventory))][0];
+
+    expect(loaded.get(loadedE, Inventory).items).toEqual([]);
+  });
+});
+
+// ── Extended: observables still work after load ──────
+describe("Serialization — observables after load", () => {
+  it("loaded world emits events on mutation", () => {
+    const w = new World();
+    w.spawn();
+
+    const loaded = roundTrip(w);
+    const events: WorldEvent[] = [];
+    loaded.events$.subscribe((e) => events.push(e));
+
+    const e = loaded.spawn();
+    expect(events).toHaveLength(1);
+    expect(events[0].type).toBe("spawned");
+  });
+
+  it("loaded world query observables work", () => {
+    const w = new World();
+    const e = w.spawn();
+    w.add(e, Position, { x: 1, y: 2 });
+
+    const loaded = roundTrip(w);
+    const updates: QueryUpdate[] = [];
+    loaded.observe(query(Position)).subscribe((u) => {
+      if (u.added.length || u.removed.length || u.changed.length) {
+        updates.push(u);
+      }
+    });
+
+    // Add a new Position entity
+    const e2 = loaded.spawn();
+    loaded.add(e2, Position, { x: 3, y: 4 });
+
+    expect(updates).toHaveLength(1);
+    expect(updates[0].added).toHaveLength(1);
+  });
+
+  it("loaded world relationship observables work", () => {
+    const w = new World();
+    const a = w.spawn();
+    const b = w.spawn();
+    w.relate(a, ChildOf, b);
+
+    const loaded = roundTrip(w);
+    const relUpdates: RelationshipUpdate[] = [];
+    loaded.observeRelated(ChildOf).subscribe((u) => relUpdates.push(u));
+
+    const c = loaded.spawn();
+    const d = loaded.spawn();
+    loaded.relate(c, ChildOf, d);
+
+    expect(relUpdates).toHaveLength(1);
+    expect(relUpdates[0].added).toHaveLength(1);
+  });
+});
+
+// ── Extended: stress ─────────────────────────────────
+describe("Serialization — stress", () => {
+  it("round-trips 500 entities with mixed components", () => {
+    const w = new World();
+
+    for (let i = 0; i < 500; i++) {
+      const e = w.spawn();
+      w.add(e, Position, { x: i, y: i * 2 });
+      if (i % 2 === 0) {
+        w.add(e, Velocity, { vx: 1, vy: 0 });
+      }
+      if (i % 3 === 0) {
+        w.add(e, Health, { current: i, max: 1000 });
+      }
+      if (i % 5 === 0) {
+        w.add(e, Name, { value: `entity_${i}` });
+      }
+    }
+
+    // Add some relationships
+    const all = [...w.query(query(Position))];
+    for (let i = 0; i < 100; i++) {
+      const src = all[i * 2];
+      const tgt = all[i * 2 + 1];
+      if (src && tgt) {
+        w.relate(src, ChildOf, tgt);
+      }
+    }
+
+    const loaded = roundTrip(w);
+    expect(loaded.entityCount).toBe(500);
+
+    const withPos = [...loaded.query(query(Position))];
+    const withVel = [...loaded.query(query(Velocity))];
+    const withHealth = [...loaded.query(query(Health))];
+
+    expect(withPos).toHaveLength(500);
+    expect(withVel).toHaveLength(250); // every 2nd
+    expect(withHealth).toHaveLength(167); // every 3rd ≈ floor(499/3)+1
+
+    // Verify a few random entities
+    const e0 = withPos[0];
+    expect(loaded.get(e0, Position).x).toBe(0);
+  });
+
+  it("repeated round-trips are idempotent", () => {
+    const w = new World();
+    const e = w.spawn();
+    w.add(e, Position, { x: 10, y: 20 });
+    w.add(e, Health, { current: 75, max: 100 });
+
+    const loaded1 = roundTrip(w);
+    const loaded2 = roundTrip(loaded1);
+
+    expect(loaded2.entityCount).toBe(loaded1.entityCount);
+
+    const e2 = [...loaded2.query(query(Position))][0];
+    expect(loaded2.get(e2, Position)).toEqual({ x: 10, y: 20 });
+    expect(loaded2.get(e2, Health)).toEqual({ current: 75, max: 100 });
+  });
+});

test/smoke.ts

@@ -1,132 +0,0 @@
-import {
-  World,
-  defineComponent,
-  query,
-  QueryUpdate,
-  WorldEvent,
-} from "../src/index";
-
-// ── Define components ─────────────────────────────────
-const Position = defineComponent({ x: 0, y: 0 });
-const Velocity = defineComponent({ vx: 0, vy: 0 });
-const Health = defineComponent({ current: 100, max: 100 });
-const Dead = defineComponent({ timestamp: 0 });
-
-// Type inference check
-const _p: { x: number; y: number } = Position.defaults;
-
-// ── World setup ──────────────────────────────────────
-const world = new World();
-
-let events: WorldEvent[] = [];
-world.events$.subscribe((e) => events.push(e));
-
-// ── Entity lifecycle ─────────────────────────────────
-const player = world.spawn();
-const enemy = world.spawn();
-
-console.assert(events.length === 2, "spawn events");
-console.assert(events[0].type === "spawned" && events[0].entity === player);
-console.assert(events[1].type === "spawned" && events[1].entity === enemy);
-
-console.assert(world.isAlive(player), "player alive");
-console.assert(world.isAlive(enemy), "enemy alive");
-console.assert(world.entityCount === 2, "two entities");
-
-// ── Add components ───────────────────────────────────
-const pos = world.add(player, Position, { x: 10, y: 20 });
-world.add(player, Velocity, { vx: 1, vy: 0 });
-world.add(enemy, Position, { x: 50, y: 0 });
-world.add(enemy, Health, { current: 50 });
-
-console.assert(pos.x === 10 && pos.y === 20, "add with init");
-console.assert(world.has(player, Position), "has Position");
-console.assert(world.has(player, Velocity), "has Velocity");
-console.assert(!world.has(player, Health), "no Health");
-console.assert(events.length === 6, "component add events");
-
-// ── Sync query ───────────────────────────────────────
-const movable = [...world.query(query(Position, Velocity))];
-console.assert(movable.length === 1, "player only in movable");
-console.assert(movable[0] === player);
-
-const allPos = [...world.query(query(Position))];
-console.assert(allPos.length === 2, "both have Position");
-
-// ── Observable query ─────────────────────────────────
-const queryLog: QueryUpdate[] = [];
-world.observe(query(Position, Velocity)).subscribe((u) => {
-  if (u.added.length || u.removed.length || u.changed.length) {
-    queryLog.push(u);
-  }
-});
-
-// ── Mutation + change tracking ───────────────────────
-world.get(player, Position).x += 5;
-world.markDirty(player, Position);
-world.get(player, Velocity).vx *= 2;
-world.markDirty(player, Velocity);
-
-// flush should emit componentChanged events and update queries
-world.flush();
-
-console.assert(
-  events.some((e) => e.type === "componentChanged"),
-  "change events",
-);
-
-// The query observer should have received changed: [player]
-const lastUpdate = queryLog[queryLog.length - 1];
-console.assert(
-  lastUpdate.changed.length === 1 && lastUpdate.changed[0] === player,
-  "player in changed",
-);
-
-// ── Remove component ─────────────────────────────────
-world.remove(player, Velocity);
-
-console.assert(!world.has(player, Velocity), "Velocity removed");
-const movableAfter = [...world.query(query(Position, Velocity))];
-console.assert(movableAfter.length === 0, "no one movable after remove");
-
-// The observer should have emitted {removed: [player]}
-const remUpdate = queryLog[queryLog.length - 1];
-console.assert(
-  remUpdate.removed.length === 1 && remUpdate.removed[0] === player,
-  "player removed from query",
-);
-
-// ── Destroy ──────────────────────────────────────────
-world.destroy(enemy);
-console.assert(!world.isAlive(enemy), "enemy destroyed");
-console.assert(world.entityCount === 1, "one entity left");
-
-// ── componentChanged query update ────────────────────
-// Add enemy back, observe query(Health).without(Dead)
-const enemy2 = world.spawn();
-world.add(enemy2, Health, { current: 75 });
-
-const healthLog: QueryUpdate[] = [];
-world.observe(query(Health).without(Dead)).subscribe((u) => {
-  healthLog.push(u);
-});
-
-// Enemy won't be in the initial seed yet (subscribe happened after spawn)
-// Let's add Dead to trigger the removal
-world.add(enemy2, Dead, { timestamp: 123 });
-world.flush();
-
-console.assert(
-  healthLog.some((u) => u.removed[0] === enemy2),
-  "enemy removed from health-not-dead query after gaining Dead",
-);
-
-// ── Entity recycling ─────────────────────────────────
-world.destroy(player);
-const recycled = world.spawn();
-
-console.assert(recycled !== player, "recycled entity has new generation");
-console.assert(world.isAlive(recycled), "recycled entity is alive");
-console.assert(!world.isAlive(player), "old handle is dead");
-
-console.log("✅ All smoke tests passed.");

test/world.test.ts

@@ -0,0 +1,361 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import {
+  World,
+  defineComponent,
+  query,
+  type QueryUpdate,
+  type WorldEvent,
+  type Entity,
+} from "../src/index";
+
+// ── Components ──────────────────────────────────────
+const Position = defineComponent("position", { x: 0, y: 0 });
+const Velocity = defineComponent("velocity", { vx: 0, vy: 0 });
+const Health = defineComponent("health", { current: 100, max: 100 });
+const Dead = defineComponent("dead", { timestamp: 0 });
+
+// ── Helpers ────────────────────────────────────────
+function collectUpdates(obs$: { subscribe: Function }): QueryUpdate[] {
+  const log: QueryUpdate[] = [];
+  obs$.subscribe((u: QueryUpdate) => {
+    if (u.added.length || u.removed.length || u.changed.length) log.push(u);
+  });
+  return log;
+}
+
+function collectEvents(world: World): WorldEvent[] {
+  const log: WorldEvent[] = [];
+  world.events$.subscribe((e: WorldEvent) => log.push(e));
+  return log;
+}
+
+// ── Entity lifecycle ───────────────────────────────
+describe("Entity lifecycle", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("spawns entities", () => {
+    const e = world.spawn();
+    expect(world.isAlive(e)).toBe(true);
+    expect(world.entityCount).toBe(1);
+  });
+
+  it("emits spawn event", () => {
+    const events = collectEvents(world);
+    const e = world.spawn();
+    expect(events).toHaveLength(1);
+    expect(events[0]).toMatchObject({ type: "spawned", entity: e });
+  });
+
+  it("destroys entities", () => {
+    const e = world.spawn();
+    world.destroy(e);
+    expect(world.isAlive(e)).toBe(false);
+    expect(world.entityCount).toBe(0);
+  });
+
+  it("emits destroy event", () => {
+    const events = collectEvents(world);
+    const e = world.spawn();
+    world.destroy(e);
+    expect(events).toHaveLength(2);
+    expect(events[1]).toMatchObject({ type: "destroyed", entity: e });
+  });
+
+  it("recycles entity indices with generation bump", () => {
+    const a = world.spawn();
+    world.destroy(a);
+    const b = world.spawn();
+    expect(b).not.toBe(a);
+    expect(world.isAlive(b)).toBe(true);
+    expect(world.isAlive(a)).toBe(false);
+  });
+
+  it("throws on operations with dead entity", () => {
+    const e = world.spawn();
+    world.destroy(e);
+    expect(() => world.get(e, Position)).toThrow("not alive");
+    expect(() => world.has(e, Position)).not.toThrow(); // has() is safe
+  });
+});
+
+// ── Components ──────────────────────────────────────
+describe("Components", () => {
+  let world: World;
+  let entity: Entity;
+
+  beforeEach(() => {
+    world = new World();
+    entity = world.spawn() as Entity;
+  });
+
+  it("add returns defaults", () => {
+    const pos = world.add(entity, Position);
+    expect(pos).toEqual({ x: 0, y: 0 });
+  });
+
+  it("add overrides defaults with init", () => {
+    const pos = world.add(entity, Position, { x: 10, y: 20 });
+    expect(pos.x).toBe(10);
+    expect(pos.y).toBe(20);
+  });
+
+  it("get returns the live mutable object", () => {
+    world.add(entity, Position, { x: 5 });
+    const pos = world.get(entity, Position);
+    pos.x = 99;
+    expect(world.get(entity, Position).x).toBe(99);
+  });
+
+  it("tryGet returns undefined when absent", () => {
+    expect(world.tryGet(entity, Position)).toBeUndefined();
+    world.add(entity, Position);
+    expect(world.tryGet(entity, Position)).toEqual({ x: 0, y: 0 });
+  });
+
+  it("has checks component presence", () => {
+    expect(world.has(entity, Position)).toBe(false);
+    world.add(entity, Position);
+    expect(world.has(entity, Position)).toBe(true);
+  });
+
+  it("remove removes the component", () => {
+    world.add(entity, Position);
+    world.remove(entity, Position);
+    expect(world.has(entity, Position)).toBe(false);
+  });
+
+  it("remove is idempotent", () => {
+    expect(() => world.remove(entity, Position)).not.toThrow();
+  });
+
+  it("set replaces and marks dirty", () => {
+    world.add(entity, Position);
+    world.set(entity, Position, { x: 42, y: 99 });
+    expect(world.get(entity, Position)).toEqual({ x: 42, y: 99 });
+  });
+
+  it("set throws if component not added first", () => {
+    expect(() => world.set(entity, Position, { x: 1, y: 2 })).toThrow(
+      "Use add()",
+    );
+  });
+
+  it("emits componentAdded event", () => {
+    const events = collectEvents(world);
+    world.add(entity, Position);
+    expect(events.find((e) => e.type === "componentAdded")).toMatchObject({
+      type: "componentAdded",
+      entity,
+    });
+  });
+
+  it("emits componentRemoved event", () => {
+    world.add(entity, Position);
+    const events = collectEvents(world);
+    world.remove(entity, Position);
+    expect(events.find((e) => e.type === "componentRemoved")).toMatchObject({
+      type: "componentRemoved",
+      entity,
+    });
+  });
+});
+
+// ── Queries ─────────────────────────────────────────
+describe("Sync queries", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("returns matching entities", () => {
+    const a = world.spawn();
+    world.add(a, Position);
+    world.add(a, Velocity);
+
+    const b = world.spawn();
+    world.add(b, Position);
+
+    const result = [...world.query(query(Position, Velocity))];
+    expect(result).toEqual([a]);
+  });
+
+  it("returns empty when no match", () => {
+    const e = world.spawn();
+    world.add(e, Position);
+    const result = [...world.query(query(Position, Velocity))];
+    expect(result).toHaveLength(0);
+  });
+
+  it("excludes with .without()", () => {
+    const a = world.spawn();
+    world.add(a, Health);
+    world.add(a, Dead);
+
+    const b = world.spawn();
+    world.add(b, Health);
+
+    const result = [...world.query(query(Health).without(Dead))];
+    expect(result).toEqual([b]);
+  });
+});
+
+// ── Observable queries ──────────────────────────────
+describe("Observable queries", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("emits added when an entity later matches", () => {
+    const log = collectUpdates(world.observe(query(Position)));
+    const e = world.spawn();
+    world.add(e, Position);
+    expect(log).toHaveLength(1);
+    expect(log[0].added).toEqual([e]);
+    expect(log[0].removed).toEqual([]);
+  });
+
+  it("emits removed when an entity stops matching", () => {
+    const e = world.spawn();
+    world.add(e, Position);
+
+    const log = collectUpdates(world.observe(query(Position)));
+    world.remove(e, Position);
+    expect(log).toHaveLength(1);
+    expect(log[0].removed).toEqual([e]);
+  });
+
+  it("emits removed on entity destroy", () => {
+    const e = world.spawn();
+    world.add(e, Position);
+
+    const log = collectUpdates(world.observe(query(Position)));
+    world.destroy(e);
+    expect(log).toHaveLength(1);
+    expect(log[0].removed).toEqual([e]);
+  });
+
+  it("emits changed on matching entities after flush", () => {
+    const e = world.spawn();
+    world.add(e, Position);
+
+    const log = collectUpdates(world.observe(query(Position)));
+    world.get(e, Position).x += 1;
+    world.markDirty(e, Position);
+    world.flush();
+
+    expect(log).toHaveLength(1);
+    expect(log[0].changed).toEqual([e]);
+  });
+
+  it("seeds with currently matching entities on subscribe", () => {
+    const e = world.spawn();
+    world.add(e, Position);
+
+    // Next subscription should know e already matches
+    const log = collectUpdates(world.observe(query(Position)));
+
+    // Remove to trigger an event
+    world.remove(e, Position);
+    expect(log).toHaveLength(1);
+    expect(log[0].removed).toEqual([e]);
+  });
+
+  it("handles .without() queries", () => {
+    const e = world.spawn();
+    world.add(e, Health);
+
+    const log = collectUpdates(world.observe(query(Health).without(Dead)));
+
+    world.add(e, Dead);
+    expect(log).toHaveLength(1);
+    expect(log[0].removed).toEqual([e]);
+  });
+});
+
+// ── Change tracking ─────────────────────────────────
+describe("Change tracking", () => {
+  let world: World;
+
+  beforeEach(() => {
+    world = new World();
+  });
+
+  it("emits componentChanged on flush", () => {
+    const e = world.spawn();
+    world.add(e, Position);
+
+    const events = collectEvents(world);
+    world.get(e, Position).x = 42;
+    world.markDirty(e, Position);
+    world.flush();
+
+    expect(events.some((ev) => ev.type === "componentChanged")).toBe(true);
+  });
+
+  it("batches multiple dirty marks into one flush", () => {
+    const a = world.spawn();
+    const b = world.spawn();
+    world.add(a, Position);
+    world.add(b, Position);
+
+    let changeCount = 0;
+    world.observe(query(Position)).subscribe((u) => {
+      changeCount += u.changed.length;
+    });
+
+    world.markDirty(a, Position);
+    world.markDirty(b, Position);
+    world.flush();
+
+    expect(changeCount).toBe(2);
+  });
+
+  it("set() implicitly marks dirty", () => {
+    const e = world.spawn();
+    world.add(e, Position);
+
+    const log = collectUpdates(world.observe(query(Position)));
+    world.set(e, Position, { x: 1, y: 2 });
+    world.flush();
+
+    expect(log).toHaveLength(1);
+    expect(log[0].changed).toEqual([e]);
+  });
+
+  it("clears dirty after flush", () => {
+    const e = world.spawn();
+    world.add(e, Position);
+
+    let changeCount = 0;
+    world.observe(query(Position)).subscribe((u) => {
+      changeCount += u.changed.length;
+    });
+
+    world.markDirty(e, Position);
+    world.flush();
+    expect(changeCount).toBe(1);
+
+    world.flush();
+    expect(changeCount).toBe(1); // no new emissions
+  });
+});
+
+// ── TypeScript inference ────────────────────────────
+describe("Type safety", () => {
+  it("infers component type from defaults", () => {
+    const Shield = defineComponent("shield", { armor: 5, broken: false });
+    const s = Shield.defaults;
+    // compile-time check: these should be the inferred types
+    const _armor: number = s.armor;
+    const _broken: boolean = s.broken;
+    expect(typeof _armor).toBe("number");
+    expect(typeof _broken).toBe("boolean");
+  });
+});

vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    include: ["test/**/*.test.ts"],
+  },
+});