Game development

This page contains information about making games for Gamedoy.

Anatomy of a game

A game is a JavaScript file that exports hooks and artwork which the app uses to start, stop and update the game.

import { Runtime } from '@robb_j/gamedoy'

interface State {
score: number

export function setup(runtime: Runtime) {
return { score: 5 } satisfies State

export function update(runtime: Runtime<State>, dt: number) {
// (optional) Game loop here

export function teardown(runtime: Runtime<State>) {
// (optional) Clean up here


The first hook, setup, is called to start your game. It should do whatever is needed to get the game going and is responsible for creating the initial game State.

You can put whatever you want in your State and it will be passed to the other hooks for easy access. For instance, you can use it to keep track of the score or certain images that have been loaded in.

setup can be asynchronous by returning a promise or using an async function, the promise will be awaited before any of the other hooks will be called. You can even finish the game within that promise if you have no need of the update hook.

This is a great time to register control handlers and setup the display of your game, all of which can be done through the runtime object which is passed to this hook.


The update hook is optional. If defined, it will be called every frame (using Window.requestAnimationFrame under the hood) until the game is finished. It is not called at all if the game is finished before setup is over. This is great place for calculating scoring, updating the State, rendering to a canvas or processing game logic.

You get the runtime object, which now has your State set on it. This makes it easy to access your State and and interact with the runtime.

You also get the dt parameter which is the number of seconds since the last update was called. This is useful for creating frame-rate-independent interactions.

Not every device will have the same frame-rate, so instead of applying constant changes each update, times any constant change by the dt to make the movement frame-rate-independent. This way the amount moved between ticks is proportional to the time elapsed so fast and slow devices all have the same experience.

The update hook must be synchronous, so any promises returned here will be ignored. It is not advised.


Once the game has finished, this optional hook is called. Use it to clean up anything you've done since the game was set up. If you've used the Runtime#disposables you don't need to clean those things up. Wherever you can use disposables, it makes everything simpler.


The Runtime is how to interact with the console itself. It has useful methods for game related things.

export interface Runtime<State, Result> {
controls: Controls
finish(result: Result)
state: State
disposables: CompositeDisposable
setDisplay(elem: HTMLElement | null): Disposable

Fields on the Runtime class lend themselves to JavaScript destructuring, such as:

export function setup({ controls, disposables, setDisplay }: Runtime) {
// ...


You'll want players to be able to interact with your game. The app provides a minimal D-pad and A/B buttons, use controls to register handlers for when these buttons are pressed, or query any buttons current state.

interface State {}

export function update({ controls }: Runtime<State>, dt: number) {
// See whether the A button is currently being pressed or not, returns a boolean
controls.state.A // boolean

There is also onKeyUp and onKeyDown to listen for when a button is pressed or released. They return a Disposable which you can use to remove the handler. This works nicely with Runtime#disposables so you don't have to unregister them in teardown. You can also use the Disposable to manually remove the handler if you need dynamic controls in your game.

interface State {}

export function setup({ controls, disposables }: Runtime) {
const state: State = {}

controls.onKeyDown('UP', () => jump(state)),
controls.onKeyUp('LEFT', () => slide(state))

return state

function jump(state: State) {
/* ... */
function slide(state: State) {
/* ... */


Every game has to end and this method triggers that. Use this to tell the app your game is over and to start the teardown process. If you want your game to animate out, you should do that before you call finish. You pass the Result of your game to this method, if you are using that feature.

interface State {}
interface Result {
score: number

export function update({ state, finish }: Runtime<State, Result>) {
if ( < 0) {
finish({ score: 42 })


Your game probably needs to remember things, like how the player is doing or what the current goal is. That's what the State is for. As mentioned in setup, state is added to the runtime for easy access and modification.


You should tidy up after yourself, disposables helps you do that. Methods like controls#onKeyUp or setDisplay return a Disposable which has a method that undoes whatever action you did. You can manually call .dispose() on that object or you can pass it to disposables to do it for you when the game is finished.

export function setup({ disposables, setDisplay, controls }: Runtime) {
controls.onKeyUp('UP', () => /* ... */)

A disposable is actually just one of these, so you can call dispose yourself on them or create your own if you like.

interface Disposable {
dispose(): void


Your game needs to look nice. So you need to give the app something to put on the display. Call setDisplay to put an HTMLElement in the app's screen for your players to see. The style of the screen makes it so the display is always square and up-to 400x400 pixels. The element you use should be a replaced element for Gamedoy's automatic sizing to work.

This returns a Disposable, which makes it easy to remove the display afterwards using disposables.

If you want to use a HTMLCanvas, see create2dCanvas in the Game Library section.

Game library

There are a set of tools and types you can use to help write your games. Everything should be imported from the mod.js file.

Yes its weird you have to import a TypeScript file with .js. It's complicated.


You can import the Runtime object from the game library which goes most of the way to writing a game in TypeScript. This helps with autocompletion and code linting. For example:

import { Runtime } from '@robb_j/gamedoy/mod.js'

interface State {
score: number
lives: number

export function setup(runtime: Runtime): State {
return { score: 0 }
export function update(runtime: Runtime<State>, dt: number) {}
export function teardown({ state }: Runtime<State>) {
return { score: state.score }


If you want to draw things, a HTMLCanvas is what you might want, and there is a helper to create one of those for you, create2dCanvas. Import it from the game lib, and call it to create a CanvasContext2D. It is an object that wraps the canvas elem, the width and height and a drawing ctx that you can use to render things. This works nicely when stored in your state.

It does a little magic to make the canvas look crisper by looking at window.devicePixelRatio and multiplying the actual size up but you only ever have to work with the width/height you pass to this method.

import {
} from '@robb_j/gamedoy/mod.js'

interface State {
canvas: CanvasContext2D

export function setup({ setDisplay, disposables }: Runtime) {
const canvas = create2dCanvas('2d', 400, 400)
return { canvas } satisfies State
export function update({ state }: Runtime<State>, dt: number) {
const { ctx, width, height } = state.canvas

ctx.clearRect(0, 0, width, height)
ctx.fillStyle = 'coral'
ctx.fillRect(150, 150, 350, 350)

More about 2d canvas rendering.

If there is interest in the future, there could be different canvas contexts like 3d or webgl.


If you want to animate something between two values over a set duration, this is your function. Call animate with the duration of your animation and a function to call at each step of the animation. The function is called with factor which is a number between 0 and 1 that represents how far through the animation it is, useful for multiplying.

import { animate } from '@robb_j/gamedoy/mod.js'

await animate(3_000, (factor) => {
// ...



For if you want to animate between values and draw to a canvas.

import { animate2dCanvas } from '@robb_j/gamedoy/mod.js'

await animate2dCanvas(canvas, 3_000, (factor) => {
// ...

The boot uses this quite a bit if you'd like inspiration. It uses animate under the hood and adds a bit to clear previous rendering and manage the ctx.


To separate concerns, you can develop your game as an iframe instead of as embedded JavaScript. Gamedoy provides these utilities to help with that.

You can create a iframeScene to launch your iframe right in the normal Gamedoy screen, then you can use ibus or iruntime within that iframe to easily interact with the scene. If you want to do things manually, createIframe details the communication between iframes & Gamedoy.


Creates a scene that shows a webpage on the screen. You can optionally pass allow to the options grant extra permissions, it's the same as the attribute on an <iframe> element.

import { iframeScene } from '@robb_j/gamedoy/mod.js'

const scene = iframeScene('', {
width: 400,
height: 400,

Run an iframe →


createIframe is used under-the-hood by iframeScene to create the <iframe> element, and bind it to the Runtime object by posting and receiving events from the frame. It returns a Disposable to remove the iframe from Gamedoy.

import { createIframe } from '@robb_j/gamedoy/mod.js'

const iframe = createIframe('', runtime, {
width: 400,
height: 400,

The messages between the scene and iframe are transmitted through an ibus, the scene sends these events to the iframe:

In return the scene listens for these events to be sent from the iframe:

When you provide the URL for your iframe to createIframe or iframeScene, you could set an extra parameter that you can detect client-side to opt-in to gamedoy features. For example:

// When creating your scene
const scene = iframeScene('', {
width: 400,
height: 400,

// Inside your iframe
const url = new URL(location.href)
if (url.searchParams.has('gamedoy')) {
const runtime = iruntime(window)
// ...


ibus creates an an event bus to communicate between two window objects using postMessage. For example the window in an iframe and the window that created the iframe. It adds a layer on top of postMessage & addEventListener to allow different types of messages to be transmitted. The bus has these methods on it:

// Inside an iframe
import { ibus } from '@robb_j/gamedoy/mod.js'

// Create a bus between the iframe's window and the window that created the iframe
const bus = ibus(window,

// Listen for key presses from the window
bus.addEventListener('onKeyDown', (payload) => {
console.log('key down', payload.key)

// At some point later, tell the scene the game is over
bus.emit('finish', { score: 7 })


iruntime is a helper to create a runtime from within an iframe. It works by using an ibus to emulate the runtime API.

import { iruntime } from '@robb_j/gamedoy'

const runtime = iruntime(window)

// Increment the score when they press A
runtime.controls.onKeyDown('A', () => {

// Finish the game when they press B
runtime.controls.onKeyUp('B', () => {
runtime.finish({ score })