feature-ecs 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

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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(). feature-ecs 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
- ecsy

📖 Usage

feature-ecs offers core ECS concepts without imposing strict rules onto your architecture:

- Entities are numerical IDs representing game objects
- Components are data containers that can follow different storage patterns
- Systems are just functions that query and process entities
- Queries provide powerful filtering with change detection

For optimal performance:

- Use Array of Structures (AoS) format for related component properties
- Implement systems as pure functions operating on query results

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``ts
import { And, createWorld, With } from 'feature-ecs';

// Define components - no registration needed!
const Position = { x: [], y: [] }; // AoS pattern
const Velocity = { dx: [], dy: [] }; // AoS pattern
const Health = []; // Single value array
const Player = {}; // Marker component

// Create world
const world = createWorld();
`

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`ts
// Create entity
const entity = world.createEntity();

// Destroy entity (removes all components)
world.destroyEntity(entity);
`

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`ts
// Add components
world.addComponent(entity, Position, { x: 100, y: 50 });
world.addComponent(entity, Velocity, { dx: 2, dy: 1 });
world.addComponent(entity, Health, 100);
world.addComponent(entity, Player, true);

// Update components (AoS)
world.updateComponent(entity, Position, { x: 110 });
world.updateComponent(entity, Health, 95);
world.updateComponent(entity, Player, false); // Also removes marker

// Direct updates - mark as changed for reactive queries
Position.x[entity] = 110;
world.markComponentChanged(entity, Position);
Health[entity] = 95;
world.markComponentChanged(entity, Health);

// Remove component
world.removeComponent(entity, Velocity);

// Check component
if (world.hasComponent(entity, Player)) {
// Entity is a player
}
`

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

// Query entity IDs
const players = world.queryEntities(With(Player));
const moving = world.queryEntities(And(With(Position), With(Velocity)));
const damaged = world.queryEntities(Changed(Health));

// Query with component data
for (const [eid, pos, health] of world.queryComponents([Entity, Position, Health] as const)) {
console.log(Entity ${eid} at (${pos.x}, ${pos.y}) with ${health} health);
}
`

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`ts
function update(deltaTime: number) {
// Movement system
for (const [eid, pos, vel] of world.queryComponents([Entity, Position, Velocity] as const)) {
world.updateComponent(eid, Position, {
x: pos.x + vel.dx * deltaTime,
y: pos.y + vel.dy * deltaTime
});
}

// Clear change tracking
world.flush();
}
`

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

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

#### Why These Decisions?

Sparse Arrays: Memory-efficient with large entity IDs - only allocated indices use memory.

Direct Array Access: No function call overhead - Health[eid] = 100 is fastest possible.

Flexible Patterns: Physics systems benefit from SoA cache locality, UI systems need complete AoS objects.

Generations: JavaScript 32-bit integers limit us to 31 components - generations provide unlimited components.

Performance: O(1) operations, 4 bytes per entity per generation, direct memory access.

📚 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