Fangorn

A simple, one-file entity component system for Typescript and Javascript in about 100 lines of code (it's where your 'ents live)

The goal of Fangorn (other than satisfying my curiosity) is to be fast enough with remaining simple, easy to use and fun to hack on.

What's an entity component system?

An entity component system, or ECS, is a way of decoupling behaviour, data and actors (or entities) in a game or other fairly complex application that needs to iterate over a lot of moving parts. There are several benefits to using this pattern in the right situation:

• Easily add or remove behaviour from an actor or entity
• Compose new and interesting behaviours and discover emergent gameplay
• Maintain reasonable performance and easily identify bottlenecks in your code

Installation

Copy the file

The only file you need is in src/ents.mts

NPM and CDN

Coming soon

Example

import { Ents } from "fangorn.mjs";
const ents = new Ents();
const PARTICLES = 10000;
const WIDTH = 480;
const HEIGHT = 320;
const CTX = (document.getElementById("canvas") as HTMLCanvasElement)?.getContext("2d");

if (!CTX) {
  throw new Error("Could not get 2d context");
}

type Vec2 = { x: number; y: number };
const vec2 = (x: number, y: number) => ({ x, y });
const randVec2 = () => vec2(Math.random() * 2 - 1, Math.random() * 2 - 1);

// Create our component storage
const Position = ents.makeComponent<Vec2>("position");
const Velocity = ents.makeComponent<Vec2>("velocity");
const Heavy = ents.makeComponent<boolean>("heavy");

for (let i = 0; i < PARTICLES; i++) {
  // Create our entities and bind component values to them
  const entity = ents.makeEntity();
  ents.bindComponent(entity, Position, vec2(WIDTH / 2, HEIGHT / 2));
  ents.bindComponent(entity, Velocity, randVec2());

  // Make half of the entities heavy
  if (Math.random() >= 0.5) {
    ents.bindComponent(entity, Heavy, true);
  }
}

const renderSystem = (ents: Ents) => {
  CTX.clearRect(0, 0, WIDTH, HEIGHT);
  // Get all entities that have a position and draw them
  for (const entity of ents.with([Position])) {
    const pos = Position[entity];
    CTX.fillRect(pos.x, pos.y, 1, 1);
  }
};

const physicsSystem = (ents: Ents) => {
  // Update the entity positions using the velocity
  for (const entity of ents.with([Position, Velocity])) {
    const pos = Position[entity];
    const vel = Velocity[entity];
    pos.x += vel.x;
    pos.y += vel.y;
    if (pos.x < 0 || pos.x >= WIDTH) {
      vel.x = -vel.x;
    }
    if (pos.y < 0 || pos.y >= HEIGHT) {
      vel.y = -vel.y;
    }
  }
};

const gravitySystem = (ents: Ents) => {
  // Make heavy entities fall and bounce
  for (const entity of ents.with<any>([Velocity, Heavy, Position])) {
    if (Velocity[entity].y < 1) {
      Velocity[entity].y += 0.01;
    }
    if (Position[entity].y > HEIGHT - 1) {
      Velocity[entity].y = -Math.abs(Velocity[entity].y) * 0.6;
    }
  }
};

document.addEventListener("DOMContentLoaded", () => {
window.setInterval(() => {
    physicsSystem(ents);
    gravitySystem(ents);
    renderSystem(ents);
}, 16);
});

How does it work?

Actors or entities are represented as ids, their properties (the components) are represented as collections of the same data type that are stored together and the entity id is used to keep track of which entity 'owns' a property. Behaviour is modelled as a system that can query for all entities with a set of components and perform updates on them.

For example, in the demo below we create 10,000 particles - each one is an entity. Each particle has a position, velocity and 'heavy' component. These componenents don't live on the entity (as they would in a more traditional object oriented model) but instead are bound using the entity id.

Our systems query for all entities with position and velocity and use the velocity to update the position. A seperate system queries for all heavy components and applies gravity.

Tests

Because testing Typescript is still a nightmare (especially with JS modules) the tests are run against the compiled Javascript in the ./dist folder. Make sure you run npx tsc or npm run build before running npm test.

TODO List

• Entity recycling: re-use ids from deleted entities
• Interactive demo
• NPM package and/or CDN delivery
• Query caching: no need to re-compute the list of entity ids for a set of components if nothing has changed
• More tests: e.g. test recycled entities don't have their old components (they don't)
• Perf tests and remove any obvious bottlenecks