ecsify is a flexible, typesafe, and performance-focused Entity Component System (ECS) library for TypeScript.

- 🔮 Simple, declarative API: Intuitive component patterns with full type safety
- 🍃 Lightweight & Tree Shakable: Function-based and modular design
- ⚡ High Performance: O(1) component checks using bitflags, cache-friendly sparse arrays
- 🔍 Powerful Querying: Query entities with complex filters and get component data efficiently
- 📦 Zero Dependencies: Standalone library ensuring ease of use in various environments
- 🔧 Flexible Storage: Supports AoS, SoA, and marker component patterns
- 🧵 Change Tracking: Built-in tracking for added, changed, and removed components

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- Basic (CodeSandbox)

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Build a modern, type-safe ECS library that fully leverages TypeScript's type system without compromising performance. While libraries like bitECS offer good speed, they often lack robust TypeScript support and more advanced queries like Added(), Removed(), or Changed(). ecsify bridges this gap - combining high performance, full TypeScript integration, and powerful query capabilities - all while adhering to the KISS principle for a clean, intuitive API.

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- bitecs
- koota
- becsy
- elics
- ecsy

📖 Usage

ecsify offers two approaches:

- App: Better DX, type safety, unified API
- Raw: Maximum performance, direct memory access

Both approaches can be mixed in the same codebase. Most users start with App. Use Raw only for performance-critical code.

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Better DX with plugins, systems, and unified API (slower than Raw because of added abstractions):

``ts
import { createApp, createDefaultPlugin, Entity, TPlugin } from 'ecsify';

// Define plugin
type TGamePlugin = TPlugin<
{
name: 'Game';
components: {
Position: { x: number[]; y: number[] };
Velocity: { dx: number[]; dy: number[] };
};
},
[TDefaultPlugin]
>;

function createGamePlugin(): TGamePlugin {
return {
name: 'Game',
deps: ['Default'],
components: {
Position: { x: [], y: [] },
Velocity: { dx: [], dy: [] }
},
setup(app) {
// Create entity
const player = app.createEntity();
app.addComponent(player, app.c.Position, { x: 0, y: 0 });
app.addComponent(player, app.c.Velocity, { dx: 1, dy: 1 });

// Register system
app.addSystem(movementSystem, { set: 'Update' });
}
};
}

function movementSystem(app: TApp>) {
for (const [eid, pos, vel] of app.queryComponents([
Entity,
app.c.Position,
app.c.Velocity
] as const)) {
// 'as const' for type inference
app.updateComponent(eid, app.c.Position, {
x: pos.x + vel.dx,
y: pos.y + vel.dy
});
}
}

// Create app
const app = createApp({
plugins: [createDefaultPlugin(), createGamePlugin()] as const, // 'as const' for type inference
systemSets: ['First', 'Update', 'Last'] // Execution order
});

// Game loop
function gameLoop() {
app.update();
requestAnimationFrame(gameLoop);
}
`

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#### Entities

Numerical IDs representing game objects:

`ts
const player = app.createEntity();
const enemy = app.createEntity();
`

#### Components

Data containers:

`ts
// Array of Structures (AoS) - simpler
const Position: { x: number; y: number }[] = [];

// Structure of Arrays (SoA) - cache-friendly
const Position: { x: number[]; y: number[] } = { x: [], y: [] };

// Single arrays
const Health: number[] = [];

// Markers
const Player = {};
`

#### Systems

Functions that process entities:

`ts
function movementSystem(app: TApp>) {
for (const [eid, pos, vel] of app.queryComponents([
Entity,
app.c.Position,
app.c.Velocity
] as const)) {
pos.x += vel.dx;
pos.y += vel.dy;
}
}

app.addSystem(movementSystem, { set: 'Update' });
`

#### Queries

Filter entities:

`ts
import { Added, And, Changed, Or, Removed, With, Without } from 'ecsify';

// Filters
app.queryEntities(With(app.c.Player)); // Has component
app.queryEntities(Without(app.c.Dead)); // Lacks component
app.queryEntities(And(With(app.c.Position), With(app.c.Velocity))); // Has all components
app.queryEntities(Or(With(app.c.Player), With(app.c.Enemy))); // Has either component

// Change tracking
app.queryEntities(Added(app.c.Player)); // Component added this frame
app.queryEntities(Removed(app.c.Velocity)); // Component removed this frame
app.queryEntities(Changed(app.c.Health)); // Component changed this frame

// Query entities with components
for (const [eid, pos, vel] of app.queryComponents(
[Entity, app.c.Position, app.c.Velocity] as const, // 'as const' for type inference
With(app.c.Player)
)) {
console.log(Player ${eid} at (${pos.x}, ${pos.y}));
}

// Query just entities
for (const eid of app.queryEntities(With(app.c.Enemy))) {
console.log(Enemy ${eid} at (${Position.x[eid]}, ${Position.y[eid]}));
}

// For reactive queries with direct updates, mark changes manually
Position.x[entity] = 110;
app.markComponentChanged(entity, Position); // Required for Changed() queries
`

#### Resources

Global state (config, input, score):

`ts
type TGamePlugin = TPlugin<
{
name: 'Game';
components: { Position: { x: number; y: number }[] };
resources: {
inputState: { jump: boolean };
gameConfig: { gravity: number };
};
},
[TDefaultPlugin]
>;

function createGamePlugin(): TGamePlugin {
return {
name: 'Game',
deps: ['Default'],
components: { Position: [] },
resources: {
inputState: { jump: false },
gameConfig: { gravity: 9.8 }
},
setup(app) {
app.addSystem(physicsSystem, { set: 'Update' });
}
};
}

function physicsSystem(app: TApp>) {
const { inputState, gameConfig } = app.r;

for (const [eid, pos] of app.queryComponents([Entity, app.c.Position] as const)) {
if (inputState.jump) pos.y -= 100;
pos.y += gameConfig.gravity;
}
}

// Modify anywhere
app.r.inputState.jump = true;
`

#### App Extensions

Custom methods on app:

`ts
type TGamePlugin = TPlugin<
{
name: 'Game';
components: {
Position: { x: number; y: number }[];
Health: number[];
Dead: Record;
};
resources: { inputState: { w: boolean; s: boolean } };
appExtensions: {
handleKeyDown: (key: string) => void;
damageEntity: (eid: TEntityId, amount: number) => void;
spawnEnemy: (x: number, y: number) => TEntityId;
};
},
[TDefaultPlugin]
>;

function createGamePlugin(): TGamePlugin {
return {
name: 'Game',
deps: ['Default'],
components: { Position: [], Health: [], Dead: {} },
resources: { inputState: { w: false, s: false } },
appExtensions: {
handleKeyDown(this: TApp>, key: string) {
if (key === 'w') this.r.inputState.w = true;
if (key === 's') this.r.inputState.s = true;
},

damageEntity(
this: TApp>,
eid: TEntityId,
amount: number
) {
const health = this.c.Health[eid];
if (health == null) return;

this.updateComponent(eid, this.c.Health, health - amount);
if (health - amount <= 0) this.addComponent(eid, this.c.Dead);
},

spawnEnemy(
this: TApp>,
x: number,
y: number
): TEntityId {
const enemy = this.createEntity();
this.addComponent(enemy, this.c.Position, { x, y });
this.addComponent(enemy, this.c.Health, 100);
return enemy;
}
},
setup(app) {
const enemy = app.spawnEnemy(100, 200);
}
};
}

// Use anywhere
app.handleKeyDown('w');
app.damageEntity(enemy, 25);
`

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`ts
const player = app.createEntity();

// Add
app.addComponent(player, app.c.Position, { x: 100, y: 50 });
app.addComponent(player, app.c.Health, 100);
app.addComponent(player, app.c.Player); // Marker

// Update
app.updateComponent(player, app.c.Position, { x: 110 }); // Partial updates (only possible for SoA)
app.updateComponent(player, app.c.Health, 95);

// Direct access (faster)
app.c.Position.x[player] = 110;
app.markComponentChanged(player, app.c.Position); // Required for Changed() queries

// Remove
app.removeComponent(player, app.c.Velocity);

// Check
if (app.hasComponent(player, app.c.Player)) {
// Is player
}
`

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`ts
function gameLoop() {
app.update(); // Runs all registered systems
requestAnimationFrame(gameLoop);
}
`

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Direct memory access for maximum performance. You manage everything manually:

`ts
import { And, createComponentRegistry, createEntityIndex, createQueryRegistry, With } from 'ecsify';

// Create registries
const entityIndex = createEntityIndex();
const componentRegistry = createComponentRegistry();
const queryRegistry = createQueryRegistry(entityIndex, componentRegistry);

// Define components
const Position: { x: number[]; y: number[] } = { x: [], y: [] };
const Velocity: { dx: number[]; dy: number[] } = { dx: [], dy: [] };

// Create entity
const player = entityIndex.createEntity();
componentRegistry.addComponent(player, Position);
Position.x[player] = 0;
Position.y[player] = 0;
componentRegistry.addComponent(player, Velocity);
Velocity.dx[player] = 1;
Velocity.dy[player] = 1;

// Systems are just functions
function movementSystem() {
for (const eid of queryRegistry.queryEntities(And(With(Position), With(Velocity)))) {
Position.x[eid] += Velocity.dx[eid];
Position.y[eid] += Velocity.dy[eid];
}
}

// Game loop
function gameLoop() {
movementSystem();
componentRegistry.flush(); // Clear change tracking
requestAnimationFrame(gameLoop);
}
`

📐 Architecture

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Efficient entity ID management using sparse-dense array pattern with optional versioning. Provides O(1) operations while maintaining cache-friendly iteration.

#### Sparse-Dense Pattern

`
Sparse Array: [_, 0, _, 2, 1, _, _] ← Maps entity ID → dense index
1 2 3 4 5 6 7 ← Entity IDs

Dense Array: [2, 5, 4, 7, 3] ← Alive entities (cache-friendly)
[0, 1, 2, 3, 4] ← Indices
└─alive─┘ └dead┘

aliveCount: 3 ← First 3 elements are alive
`

Core Data:

- Sparse Array: Maps base entity IDs to dense array positions
- Dense Array: Contiguous alive entities, with dead entities at end
- Alive Count: Boundary between alive/dead entities

#### Entity ID Format

`
32-bit Entity ID = [Version Bits | Entity ID Bits]

Example with 8 version bits:
┌─ Version (8 bits) ─┐┌─── Entity ID (24 bits) ───┐
00000001 000000000000000000000001
│ │
└─ Version 1 └─ Base Entity ID 1
`

#### Why This Design?

Problem: Stale References

`typescript
const entity = addEntity(); // Returns ID 5
removeEntity(entity); // Removes ID 5
const newEntity = addEntity(); // Might reuse ID 5!
// Bug: old reference to ID 5 now points to wrong entity
`

Solution: Versioning

`typescript
const entity = addEntity(); // Returns 5v0 (ID 5, version 0)
removeEntity(entity); // Increments to 5v1
const newEntity = addEntity(); // Reuses base ID 5 but as 5v1
// Safe: old reference (5v0) won't match new entity (5v1)
`

Swap-and-Pop for O(1) Removal

`typescript
// Remove entity at index 1:
dense = [1, 2, 3, 4, 5];
// 1. Swap with last: [1, 5, 3, 4, 2]
// 2. Decrease alive count
// Result: [1, 5, 3, 4 | 2] - only alive section matters
`

Performance: O(1) all operations, ~8 bytes per entity, cache-friendly iteration.

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Entity filtering with two strategies: bitmask optimization for simple queries, individual evaluation for complex queries.

#### Query Filters

`typescript
// Component filters
With(Position); // Entity must have component
Without(Dead); // Entity must not have component

// Change detection
Added(Position); // Component added this frame
Changed(Health); // Component modified this frame
Removed(Velocity); // Component removed this frame

// Logical operators
And(With(Position), With(Velocity)); // All must match
Or(With(Player), With(Enemy)); // Any must match
`

#### Evaluation Strategies

Bitmask Strategy - Fast bitwise operations:

`typescript
// Components get bit positions
Position: bitflag=0b001, Velocity: bitflag=0b010, Health: bitflag=0b100

// Entity masks show what components each entity has
entity1: 0b011 // Has Position + Velocity
entity2: 0b101 // Has Position + Health

// Query: And(With(Position), With(Velocity)) → andMasks.with = 0b011
// Check: (entityMask & 0b011) === 0b011
entity1: (0b011 & 0b011) === 0b011 ✓ true
entity2: (0b101 & 0b011) === 0b011 ✗ false
`

Individual Strategy - Per-filter evaluation for complex queries:

`typescript
// Complex queries like Or(With(Position), Changed(Health))
// Fall back to: filters.some(filter => filter.evaluate(app, eid))
`

#### Performance (10,000 entities)

`
individual + cached - __tests__/query.bench.ts > Query Performance > With(Position)
1.04x faster than bitmask + cached
7.50x faster than bitmask + no cache
7.83x faster than individual + no cache

bitmask + cached - __tests__/query.bench.ts > Query Performance > And(With(Position), With(Velocity))
1.01x faster than individual + cached
13.58x faster than bitmask + no cache
13.72x faster than individual + no cache
`

Key Insight: Caching matters most (7-14x faster than no cache). Bitmask vs individual evaluation shows minimal difference.

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Component management with direct array access, unlimited components via generations, and flexible storage patterns.

#### Component Patterns

`typescript
// Array of Structures (AoS) - good for complete entity data
const Transform = [];
Transform[eid] = { x: 10, y: 20 };

// Structure of Arrays (SoA) - cache-friendly for bulk operations
const Position = { x: [], y: [] };
Position.x[eid] = 10;
Position.y[eid] = 20;

// Single arrays and marker components
const Health = []; // Health[eid] = 100
const Player = {}; // Just presence/absence
`

#### Generation System

Unlimited components beyond 31-bit limit:

Why Generations? Bitmasks need one bit per component for fast O(1) checks. JavaScript integers are 32-bit, giving us only 31 usable bits (0 - 30, bit 31 is sign). So we can only track 31 components per bitmask.

`typescript
// Problem: Only 31 components fit in one integer bitmask
// Bits: 31 30 29 28 ... 3 2 1 0
// Components: ❌ ✓ ✓ ✓ ... ✓ ✓ ✓ ✓ (31 components max)

// Solution: Multiple generations, each with 31 components
// Generation 0: Components 0-30 (bitflags 1, 2, 4, ..., 2^30)
Position: { generationId: 0, bitflag: 0b001 }
Velocity: { generationId: 0, bitflag: 0b010 }

// Generation 1: Components 31+ (bitflags restart)
Armor: { generationId: 1, bitflag: 0b001 }
Weapon: { generationId: 1, bitflag: 0b010 }

// Entity masks stored per generation
_entityMasks[0][eid] = 0b011; // Has Position + Velocity
_entityMasks[1][eid] = 0b001; // Has Armor
`

#### Bitmask Operations

`typescript
// Adding component: OR with bitflag
entityMask |= 0b010; // Add Velocity

// Removing component: AND with inverted bitflag
entityMask &= ~0b010; // Remove Velocity

// Checking component: AND with bitflag
const hasVelocity = (entityMask & 0b010) !== 0;
`

#### Change Tracking

`typescript
// Separate masks track changes per frame
_addedMasks[0][eid] |= bitflag; // Component added
_changedMasks[0][eid] |= bitflag; // Component changed
_removedMasks[0][eid] |= bitflag; // Component removed

// Clear at frame end
flush() { / clear all change masks / }
`

📚 Good to Know

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JavaScript sparse arrays store only assigned indices, making them memory-efficient:

`ts
const sparse = [];
sparse[1000] = 5; // [<1000 empty items>, 5]

console.log(sparse.length); // 1001
console.log(sparse[500]); // undefined (no memory used)
`

In contrast, dense arrays allocate memory for every element, even if unused:

`ts
const dense = new Array(1001).fill(0); // Allocates 1001 × 4 bytes = ~4KB

console.log(dense.length); // 1001
console.log(dense[500]); // 0
``

Use sparse arrays for large, mostly empty datasets. Use dense arrays when you need consistent iteration and performance.

💡 Resources / References

- BitECS - High-performance ECS library that inspired our implementation
- Bevy - Data-driven game engine built in Rust that inspired our API
- Data Oriented Design and Entity Component System Explained