Entity

An Entity is a container with a unique ID that holds Components and tags. You don't construct entities directly — the World does.

Creating an entity

const player = world.createEntity('Player');

The string argument is a logical name — it appears in scene JSON, in debug panels, and helps when reading test failures. It's not required to be unique. The Entity also gets an auto-assigned id (a UUID) which is unique.

console.log(player.name);   // 'Player'
console.log(player.id);     // 'ent_a7b3c9...'

Adding and getting components

import { Component } from '@babylonjsmarket/ecs';

class Health extends Component {
  current = 100;
  max = 100;
}

player.add(new Health());

const hp = player.get(Health);              // Health | undefined
const hp2 = player.get(Health)!;             // Health (non-null assertion)
const has = player.has(Health);             // boolean

get() returns undefined when the component isn't on the entity, so the type system forces you to handle the missing case. The entity.get(X)! non-null assertion is the shorter form when you've already filtered in a System query — you know the component is there because the query required it.

Removing components

player.remove(Health);  // returns true if removed, false if not present

When a Component is removed, its onDetachOverride() hook fires (if defined) and any System whose query depended on that Component automatically drops the entity from its working set.

Tags

Tags are lightweight string markers for fast filtering. Use them when you don't need a full Component — when the presence of a label is the entire signal.

player.addTag('player');
player.addTag('alive');

player.hasTag('alive');       // true
player.removeTag('alive');
player.getTags();              // ['player']

Tags are first-class in System queries:

class AISystem extends System {
  query: ISystemQuery = {
    required: [BrainComponent],
    tags: ['enemy', 'alive'],   // entity must have BOTH tags
  };
}

Use tags for category-style filters ('player', 'enemy', 'projectile', 'pickup') and Components for data-bearing state.

Destroying an entity

world.destroyEntity(player);
// or
world.destroyEntityById(player.id);

Destroying:

1. Fires each Component's onDetachOverride() hook
2. Removes the entity from every System's working set
3. Emits 'entity.destroyed' on the EventBus
4. Releases the entity ID

Systems can subscribe to 'entity.destroyed' to clean up renderer handles or external references they hold. The EntityEvents constant gives you the event name without typos:

import { EntityEvents } from '@babylonjsmarket/ecs';

this.eventBus.on(EntityEvents.DESTROYED, ({ entityId }) => {
  // clean up
});

Finding entities

The World maintains the master entity list. Most lookups go through it:

world.getEntityById('ent_a7b3c9...');
world.getEntitiesByName('Player');         // [] of any entities named 'Player'
world.getEntitiesByTag('alive');
world.getAllEntities();

For per-frame iteration, prefer Systems — system.entities is a pre-computed working set that respects the query. Looping over world.getAllEntities() every frame defeats the point of an ECS.

Entity ID stability

Entity IDs are stable across world.update() calls. They're also stable across save/load — the SaveLoad component serializes the IDs along with the rest of the state, so a restored World restores its references.

If you need to refer to an entity across frames, store the ID, not the Entity. The Entity object may be garbage-collected if destroyed; the ID is a string.

Where to next

• System — how Systems pick up the entities they care about
• World — entity creation/destruction, batching, ticking