JavaScript, Events, DOM APIs

Note: This documentation is for the old 1.5.0 version of A-Frame. Check out the documentation for the current 1.6.0 version

Since A-Frame is just HTML, we can control the scene and its entities using JavaScript and DOM APIs as we mostly would in ordinary web development.

With JavaScript Image by Ruben Mueller from The VR Jump.

Every element in the scene, even elements such as <a-box> or <a-sky>, are entities (represented as <a-entity>). A-Frame modifies the HTML element prototype to add some extra behavior for certain DOM APIs to tailor them to A-Frame. See the Entity API documentation for reference on most of the APIs discussed below.

Where to Place JavaScript Code for A-Frame

Important: Before we go over the different ways to use JavaScript and DOM APIs, we prescribe encapsulating your JavaScript code within A-Frame components. Components modularize code, make logic and behavior visible from HTML, and ensure that code is executed at the correct time (e.g., after the scene and entities have attached and initialized). As the most basic example, to register a console.log component before <a-scene>:

AFRAME.registerComponent('log', {
  schema: {type: 'string'},

  init: function () {
    var stringToLog = this.data;
    console.log(stringToLog);
  }
});

And after the registration, use the component from HTML:

<a-scene log="Hello, Scene!">
  <a-box log="Hello, Box!"></a-box>
</a-scene>

Components encapsulate all of our code to be reusable, declarative, and shareable. Though if we’re just poking around at runtime, we can use our browser’s Developer Tools Console to run JavaScript on our scene.

Do not try to put A-Frame-related JavaScript in a raw <script> tag after <a-scene> as we would with traditional 2D scripting. If we do, we’d have to take special measures to make sure code runs at the right time (see Running Content Scripts on the Scene).

Getting Entities by Querying and Traversing

The wonderful thing about the DOM as a scene graph is that the standard DOM provides utilities for traversal, querying, finding, and selecting through .querySelector() and .querySelectorAll(). Originally inspired by jQuery selectors, we can learn about query selectors on MDN.

Let’s run a few example query selectors. Take the scene below for example.

<html>
  <a-scene>
    <a-box id="redBox" class="clickable" color="red"></a-box>
    <a-sphere class="clickable" color="blue"></a-sphere>
    <a-box color="green"></a-box>
    <a-entity light="type: ambient"></a-entity>
    <a-entity light="type: directional"></a-entity>
  </a-scene>
</html>

With `.querySelector()`

If we want to grab just one element, we use .querySelector() which returns one element. Let’s grab the scene element:

var sceneEl = document.querySelector('a-scene');

Note if we were working within a component, we’d already have a reference to the scene element without needing to query. All entities have reference to their scene element:

AFRAME.registerComponent('foo', {
  init: function () {
    console.log(this.el.sceneEl);  // Reference to the scene element.
  }
});

If an element has an ID, we can use an ID selector (i.e., #<ID>). Let’s grab the red box which has an ID. Before we did a query selector on the entire document. Here, we’ll do a query selector just within the scope of the scene. With query selectors, we’re able to limit the scope of the query to within any element:

var sceneEl = document.querySelector('a-scene');
console.log(sceneEl.querySelector('#redBox'));
// <a-box id="redBox" class="clickable" color="red"></a-box>

With `.querySelectorAll()`

If we want to grab a group of elements, we use .querySelectorAll() which returns an array of elements. We can query across element names:

console.log(sceneEl.querySelectorAll('a-box'));
// [
//  <a-box id="redBox" class="clickable" color="red"></a-box>,
//  <a-box color="green"></a-box>
// ]

We can query for elements that have a class with a class selector (i.e., .<CLASS_NAME>). Let’s grab every entity that has the clickable class:

console.log(sceneEl.querySelectorAll('.clickable'));
// [
//  <a-box id="redBox" class="clickable" color="red"></a-box>
//  <a-sphere class="clickable" color="blue"></a-sphere>
// ]

We can query for elements containing an attribute (or in this case, a component) with an attribute selector (i.e., [<ATTRIBUTE_NAME>]). Let’s grab every entity that has a light:

console.log(sceneEl.querySelectorAll('[light]'));
// [
//  <a-entity light="type: ambient"></a-entity>
// <a-entity light="type: directional"></a-entity>
// ]

Looping Over Entities from `.querySelectorAll()`

If we grabbed a group of entities using .querySelectorAll(), we can loop over them with a for loop. Let’s loop over every element in the scene with *.

var els = sceneEl.querySelectorAll('*');
for (var i = 0; i < els.length; i++) {
  console.log(els[i]);
}

A Note About Performance

Avoid using .querySelector and .querySelectorAll in tick and tock functions that get called every frame as it does take some time to loop over the DOM to retrieve entities. Instead, keep a cached list of entities, calling the query selectors beforehand, and then just loop over that.

AFRAME.registerComponent('query-selector-example', {
  init: function () {
    this.entities = document.querySelectorAll('.box');
  },
  
  tick: function () {
    // Don't call query selector in here, query beforehand.
    for (let i = 0; i < this.entities.length; i++) {
      // Do something with entities.
    }
  }
});

Retrieving Component Data with `.getAttribute()`

We can get the data of components of an entity via .getAttribute. A-Frame augments .getAttribute to return values rather than strings (e.g., returning objects in most cases since components usually consist of multiple properties, or returning an actual boolean for like .getAttribute('visible'). Often, .getAttribute will return the internal data object of the component so do not modify the object directly:

// <a-entity geometry="primitive: sphere; radius: 2"></a-entity>
el.getAttribute('geometry');
// >> {"primitive": "sphere", "radius": 2, ...}

Retrieving `position` and `scale`

Doing el.getAttribute('position') or el.getAttribute('scale') will return the three.js Object3D position and scale properties which are Vector3s. Keep in mind that modifying these objects will modify the actual entity data.

This is because A-Frame allows us to modify position, rotation, scale, visible at the three.js level, and in order for .getAttribute to return the correct data, A-Frame returns the actual three.js Object3D objects.

This is not true for the .getAttribute('rotation') because A-Frame, for better or worse, uses degrees instead of radians. In such case, a normal JavaScript object with x/y/z properties is returned. The Object3D Euler can be retrieved via el.object3D.rotation if we need to work at a lower level with radians.

Modifying the A-Frame Scene Graph

With JavaScript and DOM APIs, we can dynamically add and remove entities as we would with normal HTML elements.

Creating an Entity with `.createElement()`

To create an entity, we can use document.createElement. This will give us a blank entity:

var el = document.createElement('a-entity');

However, this entity will not be initialized or be a part of the scene until we attach it to our scene.

Adding an Entity with `.appendChild()`

To add an entity to the DOM, we can use .appendChild(element). Specifically, we want to add it to our scene. We grab the scene, create the entity, and append the entity to our scene.

var sceneEl = document.querySelector('a-scene');
var entityEl = document.createElement('a-entity');
// Do `.setAttribute()`s to initialize the entity.
sceneEl.appendChild(entityEl);

Note that .appendChild() is an asynchronous operation in the browser. Until the entity has finished appending to the DOM, we can’t do many operations on the entity (such as calling .getAttribute()). If we need to query an attribute on an entity that has just been appended, we can listen to the loaded event on the entity, or place logic in an A-Frame component so that it is executed once it is ready:

var sceneEl = document.querySelector('a-scene');

AFRAME.registerComponent('do-something-once-loaded', {
  init: function () {
    // This will be called after the entity has properly attached and loaded.
    console.log('I am ready!');
  }
});

var entityEl = document.createElement('a-entity');
entityEl.setAttribute('do-something-once-loaded', '');
sceneEl.appendChild(entityEl);

Removing an Entity with `.removeChild()`

To remove an entity from the DOM and thus from the scene, we call .removeChild(element) from the parent element. If we have an entity, we have to ask its parent (parentNode) to remove the entity.

entityEl.parentNode.removeChild(entityEl);

Modifying an Entity

A blank entity doesn’t do anything. We can modify the entity by adding components, configuring component properties, and removing components.

Adding a Component with `.setAttribute()`

To add a component, we can use .setAttribute(componentName, data). Let’s add a geometry component to the entity.

entityEl.setAttribute('geometry', {
  primitive: 'box',
  height: 3,
  width: 1
});

Or adding the community physics component:

entityEl.setAttribute('dynamic-body', {
  shape: 'box',
  mass: 1.5,
  linearDamping: 0.005
});

Unlike a normal HTML .setAttribute(), an entity’s .setAttribute() is improved to take a variety of types of arguments such as objects, or to be able to update a single property of a component. Read more about Entity.setAttribute().

Updating a Component with `.setAttribute()`

To update a component, we also use .setAttribute(). Updating a component takes several forms.

Updating Property of Single-Property Component

Let’s update the property of the position component, a single-property component. We can pass either an object or a string. It is slightly preferred to pass an object so A-Frame doesn’t have to parse the string.

entityEl.setAttribute('position', {x: 1, y: 2, z: -3});
// Read on to see why `entityEl.object3D.position.set(1, 2, -3)` is preferred though.

Updating Single Property of Multi-Property Component

Let’s update a single property of the material component, a multi-property component. We do this by providing the component name, property name, and then property value to .setAttribute():

entityEl.setAttribute('material', 'color', 'red');

Updating Multiple Properties of a Multi-Property Component

Let’s update multiple properties at once of the light component, a multi-property component. We do this by providing the component name and an object of properties to .setAttribute(). We’ll change the light’s color and intensity but leave the type the same:

// <a-entity light="type: directional; color: #CAC; intensity: 0.5"></a-entity>
entityEl.setAttribute('light', {color: '#ACC', intensity: 0.75});
// <a-entity light="type: directional; color: #ACC; intensity: 0.75"></a-entity>

Updating `position`, `rotation`, `scale`, and `visible`.

As a special case, for better performance, memory, and access to utilities, we recommend modifying position, rotation, scale, and visible directly at the three.js level via the entity’s Object3D rather than via .setAttribute:

// Examples for position.
entityEl.object3D.position.set(1, 2, 3);
entityEl.object3D.position.x += 5;
entityEl.object3D.position.multiplyScalar(5);

// Examples for rotation.
entityEl.object3D.rotation.y = THREE.MathUtils.degToRad(45);
entityEl.object3D.rotation.divideScalar(2);

// Examples for scale.
entityEl.object3D.scale.set(2, 2, 2);
entityEl.object3D.scale.z += 1.5;

// Examples for visible.
entityEl.object3D.visible = false;
entityEl.object3D.visible = true;

This lets us skip over the .setAttribute overhead and instead do simple setting of properties for components that are most commonly updated. Updates at the three.js level will still be reflected when doing for example entityEl.getAttribute('position');.

Replacing Properties of a Multi-Property Component

Let’s replace all the properties of the geometry component, a multi-property component. We do this by providing the component name, an object of properties to .setAttribute(), and a flag that specifies to clobber the existing properties. We’ll replace all of the geometry’s existing properties with new properties:

// <a-entity geometry="primitive: cylinder; height: 4; radius: 2"></a-entity>
entityEl.setAttribute('geometry', {primitive: 'torusKnot', p: 1, q: 3, radiusTubular: 4}, true);
// <a-entity geometry="primitive: torusKnot; p: 1; q: 3; radiusTubular: 4"></a-entity>

Removing a Component with `.removeAttribute()`

To remove or detach a component from an entity, we can use .removeAttribute(componentName). Let’s remove the default wasd-controls from the camera entity:

var cameraEl = document.querySelector('[camera]');
cameraEl.removeAttribute('wasd-controls');

Events and Event Listeners

With JavaScript and the DOM, there is an easy way for entities and components to communicate with one another: events and event listeners. Events are a way to send out a signal that other code can pick up and respond to. Read more about browser events.

Emitting an Event with `.emit()`

A-Frame elements provide an easy way to emit custom events with .emit(eventName, eventDetail, bubbles). For example, let’s say we are building a physics component and we want the entity to send out a signal when it has collided with another entity:

entityEl.emit('physicscollided', {collidingEntity: anotherEntityEl}, false);

Then other parts of the code can wait and listen on this event and run code in response. We can pass information and data through the event detail as the second argument. And we can specify whether the event bubbles, meaning that the parent entities will also emit the event. So other parts of the code can register an event listener.

Adding an Event Listener with `.addEventListener()`

Like with normal HTML elements, we can register an event listener with .addEventListener(eventName, function). When the event the listener is registered to is emitted, then the function will be called and handle the event. For example, continuing from the previous example with the physics collision event:

entityEl.addEventListener('physicscollided', function (event) {
  console.log('Entity collided with', event.detail.collidingEntity);
});

When the entity emits the physicscollided event, the function will be called with the event object. Notably in the event object, we have the event detail which contains data and information passed through the event.

Removing an Event Listener with `.removeEventListener()`

Like with normal HTML elements, when we want to remove the event listener, we can use .removeEventListener(eventName, function). We have to pass the same event name and function that the listener was registered with. For example, continuing from the previous example with the physics collision event:

// We have to define this function with a name if we later remove it.
function collisionHandler (event) {
  console.log('Entity collided with', event.detail.collidingEntity);
}

entityEl.addEventListener('physicscollided', collisionHandler);
entityEl.removeEventListener('physicscollided', collisionHandler);

Binding Event Listeners

By default, Javascript execution context rules binds this to the global context (window) for any independent function, meaning that these functions won’t have access to the component’s this by default.

In order for the component’s this to be accessible inside an event listener, it must be bound.

There are several ways you can do this:

By using an arrow function to define the event listener. Arrow functions automatically bind this

this.el.addEventListener('physicscollided', (event) => {
    console.log(this.el.id);
});

By defining your event listener within the events object of the component (this will also handling adding and removing the listener automatically)

See the explanation here.
By creating another function, which is the bound version of the function.

this.listeners = {
    clickListener: this.clickListener.bind(this);
}
entityEl.addEventListener('click', this.listeners.clickListener);

Caveats

A-Frame entities and primitives are implemented in a way that favours performance such that some HTML APIs may not work as expected. For instance, attribute selectors involving values won’t work and a mutation observer won’t trigger changes when a entity’s component is altered.

A-Frame

JavaScript, Events, DOM APIs

Where to Place JavaScript Code for A-Frame

Getting Entities by Querying and Traversing

With .querySelector()

With .querySelectorAll()

Looping Over Entities from .querySelectorAll()

A Note About Performance

Retrieving Component Data with .getAttribute()

Retrieving position and scale