Script Components

Writing Script Components for Creator Hub

This document explains how to write .tsx files that are used inside a Script component on an entity in a Creator Hub scene. These scripts run as self-contained classes attached to individual entities.

Where script files must live

Script files referenced by a Script component MUST live inside the assets/ folder — use assets/scripts/ (e.g. assets/scripts/MyScript.ts). Do NOT place them in a root-level scripts/ folder.

Why: the scene's tsconfig.json only includes src/**/* and assets/**/*:

"include": ["src/**/*.ts", "src/**/*.tsx", "assets/**/*.ts", "assets/**/*.tsx"]

A file in a root-level scripts/ folder falls outside these globs, so the TypeScript type checker (and the build's type-check step) won't cover it. The Script component's path field is resolved relative to the project root, so it must match the real file location — e.g. "path": "assets/scripts/SitChair.ts".

Script structure

Every script is a single exported class with:

• A constructor that receives configurable parameters (exposed in the Creator Hub UI).
• An optional start() method, called once when the scene loads.
• An optional update(dt: number) method, called every frame (~30 FPS; see "When scripts run" below for ordering — by default scripts update after all regular systems).

The first two constructor parameters must always be public src: string and public entity: Entity — do not remove or reorder them.

import { engine, Entity, Transform } from '@dcl/sdk/ecs'

export class MyScript {
  constructor(
    public src: string,
    public entity: Entity,
    public speed: number = 1
  ) {}

  start() {
    console.log('Script started on entity:', this.entity)
  }

  update(dt: number) {
    const transform = Transform.getMutable(this.entity)
    transform.rotation.y += this.speed * dt
  }
}

When scripts run — the priority field (IMPORTANT)

The Creator Hub Script component has a priority field (separate from constructor params — it's set in the component UI, not in your class). It defaults to 0, and this has non-obvious consequences:

• Default priority 0 = scripts run LAST each frame. At build time @dcl/sdk-commands groups all scripts by their priority value and registers ONE engine system per group via engine.addSystem(updateLoop, Number(priority)). Because engine systems run highest-priority-first (regular systems use 100000, UI uses 100000), priority 0 runs after everything else.
• All scripts sharing the same priority share ONE system callback and run sequentially inside it. A single heavy script's update() therefore delays every other script in the same priority group that frame.
• To run a script's update() before regular systems, raise its priority (e.g. 1000000) in the Script component. This also gives it its own dedicated system. This is how you'd step a physics library (cannon.js) inside update(dt) ahead of systems that read the result — there is no separate physics loop.

Higher priority number = earlier execution — the OPPOSITE of the "priority 1 = first" assumption. See the scene-runtime skill's "System Execution Order & Priority" section for the underlying engine rule.

Constructor parameters

Parameters declared in the constructor are exposed in the Creator Hub UI and can be configured per-entity. Allowed types:

• string
• number
• boolean
• Entity (lets the user pick another entity from the scene)

Both public and private parameters are exposed to Creator Hub. Use this.<paramName> to access values in your code.

Default values

Provide default values so the script works out of the box:

constructor(
  public src: string,
  public entity: Entity,
  public radius: number = 5,
  public label: string = 'Hello',
  public enabled: boolean = true,
) {}

Optional parameters

Use ? for parameters that may be left empty by the user:

constructor(
  public src: string,
  public entity: Entity,
  public targetEntity?: Entity,
  public message?: string,
) {}

Parameter tooltips

Add @param annotations in a JSDoc comment block directly before the constructor to show tooltips in the Creator Hub UI:

/**
 * @param startDate - The start date of the campaign in YYYY-MM-DD format
 * @param endDate - The end date of the campaign in YYYY-MM-DD format
 * @param wearableYOffset - How many meters above the ground the wearable should be displayed
 */
constructor(
  public src: string,
  public entity: Entity,
  public startDate?: string,
  public endDate?: string,
  public wearableYOffset: number = 0.5,
) {}

The layout field — how params are stored and resolved at runtime

The asset-packs::Script component value has the shape { value: [{ path: string, priority: number, layout?: string }] }. The layout field is a JSON string:

{"params":{"paramName":{"type":"string|number|boolean|action","value":<value>}}}

At runtime, @dcl/sdk-commands/dist/logic/runtime-script.js resolves params like this:

1. JSON.parse(layout) to get a ScriptLayout object.
2. Object.values(layout.params) extracts param values in insertion order.
3. The values are spread positionally into the constructor: new ScriptClass(src, entity, ...params).

CRITICAL gotcha — params are positional, not named. The key insertion order in the layout JSON MUST match the constructor parameter order (after src and entity). You cannot skip earlier params to set later ones — every param from src/entity onward must be present in order.

Params with type: "action" have value: {entity: Entity, action: string} and are converted to ActionCallback functions via createActionCallback (which calls getActionEvents(entity).emit(action, {}) when invoked).

This is relevant for agents/MCP tools setting Script params programmatically via set_component on asset-packs::Script — the full ordered params object must be provided. (When the scene is open in the Creator Hub, prefer its MCP attach_script tool — write the script file, then call it with the path and it builds the component for you; see creator-hub-mcp.) Example layout for a script with 14 constructor params (after src/entity):

{"params":{"colorA":{"type":"string","value":"#ff33e6"},"colorB":{"type":"string","value":"#33e6ff"},"rate":{"type":"number","value":35},"lifetime":{"type":"number","value":2.5},"sizeMin":{"type":"number","value":0.3},"sizeMax":{"type":"number","value":0.6},"speedMin":{"type":"number","value":2.5},"speedMax":{"type":"number","value":4},"gravity":{"type":"number","value":-0.3},"spin":{"type":"number","value":0},"shape":{"type":"string","value":"sphere"},"shapeSize":{"type":"number","value":0.4},"maxParticles":{"type":"number","value":150},"startPlaying":{"type":"boolean","value":true}}}

Verified in: @dcl/sdk-commands/dist/logic/runtime-script.js — resolveScriptParams uses Object.values(params).map(...) and the result is spread as new ScriptClass(src, entity, ...params).

Referencing assets with this.src

If your script uses additional assets that are only loaded via code (sound files, textures, models, etc.), they won't be automatically included in the custom item folder. You must add those files manually.

Always use this.src to build the path to bundled asset files, because the actual file location may differ when the item is used in another scene:

import { AudioSource } from '@dcl/sdk/ecs'

start() {
  AudioSource.create(this.entity, {
    audioClipUrl: this.src + '/sounds/click.mp3',
    playing: false
  })
}

Referencing child entities

Do not pass entities that belong to the same custom item as Entity input parameters. Entity IDs are not stable across scenes — an entity ID that is valid in your development scene may not exist in a user's scene.

Instead, find child entities at runtime by iterating over the entity hierarchy and matching their Name component value with a substring match (e.g. .startsWith(...) or .includes(...)).

Best practice: match a substring of the child's name, not an exact name, and not a per-instance constructor parameter. When a user duplicates a smart item or drops multiple copies into a scene, the Creator Hub editor auto-numbers the duplicated child entities — e.g. "Sit Spot", "Sit Spot 2", "Sit Spot 3", … A substring match (name.includes('Sit Spot')) finds the matching child in every copy automatically, with zero per-instance configuration.

• Exact-name match fails on every duplicate after the first, because their names are auto-suffixed.
• A constructor string/Entity parameter for the child name forces the user to manually rename or rewire each copy, which defeats the purpose of a reusable scripted item.

The example below uses .startsWith('Needle') — a substring-style match — for exactly this reason.

import { engine, Entity, Transform, Name } from '@dcl/sdk/ecs'

export class ClapMeter {
  private needleEntities: Entity[] = []

  constructor(
    public src: string,
    public entity: Entity,
  ) {}

  start() {
    for (const [childEntity, transform] of engine.getEntitiesWith(Transform)) {
      if (transform.parent === this.entity) {
        const nameComponent = Name.getOrNull(childEntity)
        if (nameComponent && nameComponent.value.startsWith('Needle')) {
          this.needleEntities.push(childEntity)
        }
      }
    }
  }
}

This pattern keeps the script portable: as long as the child entities have names containing the expected substring, it works in any scene and across any number of duplicated copies.

Defining actions (@action)

If your script has functions that could be useful to call from other items in the scene, mark them by adding a JSDoc comment block (/** ... */) with an @action tag directly before the method. Then add an Action component to the entity and define a corresponding action. This lets other smart items (e.g. a button) pick and trigger this action.

CRITICAL: Use ONLY the @action JSDoc tag — NEVER decorator syntax (@action() above the method). The Creator Hub parser has no decorators plugin, so a decorator makes parsing fail and ALL params and actions silently disappear from the UI.

An optional description line before the @action tag becomes the action's description shown in the Creator Hub UI.

import { engine, Entity } from '@dcl/sdk/ecs'

export class TreasureChest {
  private isOpen = false

  constructor(
    public src: string,
    public entity: Entity,
  ) {}

  /**
   * Opens the chest
   * @action
   */
  open() {
    if (this.isOpen) return
    this.isOpen = true
    console.log('Chest opened!')
  }

  /**
   * Closes the chest
   * @action
   */
  close() {
    if (!this.isOpen) return
    this.isOpen = false
    console.log('Chest closed!')
  }
}

With the @action JSDoc tag, open and close become available in the Actions component dropdown and can be triggered by other smart items or scripts.

ActionCallback parameters

Use the ActionCallback type from ~sdk/script-utils to let users wire up editor-configured actions as callbacks on your script. The user can then assign any action (from any item) to that callback in the Creator Hub UI.

import { Entity } from '@dcl/sdk/ecs'
import type { ActionCallback } from '~sdk/script-utils'

export class Padlock {
  constructor(
    public src: string,
    public entity: Entity,
    public onUnlock: ActionCallback,
  ) {}

  /**
   * @action
   */
  solve() {
    this.onUnlock()
  }
}

Calling other scripts from code

Use the runtime utilities in ~sdk/script-utils to call methods on other Script component instances:

import {
  callScriptMethod,
  getScriptInstance,
  getAllScriptInstances,
  getScriptInstancesByPath
} from '~sdk/script-utils'

callScriptMethod(entity, 'assets/scripts/Padlock.ts', 'solve', 123)

const instance = getScriptInstance(entity, 'assets/scripts/Padlock.ts')
const allOnEntity = getAllScriptInstances(entity)
const allByPath = getScriptInstancesByPath('assets/scripts/Padlock.ts')