JS Paint

A pixel-perfect web-based MS Paint remake and more... Try it out! Then join the Discord server to share your art!

JS Paint recreates every tool and menu of MS Paint, and even little-known features, to a high degree of fidelity.

It supports themes, additional file types, and accessibility features like Eye Gaze Mode and Speech Recognition.

Ah yes, good old Paint. Not the one with the ribbons or the new skeuomorphic one with the interface that can take up nearly half the screen. (And not the even newer Paint 3D.)

Windows 95, 98, and XP were the golden years of Paint. You had a tool box and a color box, a foreground color and a background color, and that was all you needed.

Things were simple.

But we want to undo more than three actions. We want to edit transparent images. We can't just keep using the old Paint.

So that's why I'm making JS Paint. I want to bring good old Paint into the modern era.

Current improvements include:

• Open source (MIT licensed)
• Cross-platform
• Mobile friendly
  • Touch support: use two fingers to pan the view, and pinch to zoom
  • Click/tap the selected colors area to swap the foreground and background colors
  • View > Fullscreen to toggle fullscreen mode, nice for small screens
• Web features
  • File > Load From URL... to open an image from the Web.
  • File > Upload to Imgur to upload the current image to Imgur.
  • Paste supports loading from URLs.
  • You can create links that will open an image from the Web in JS Paint. For example, this link will start with an isometric grid as a template: https://jspaint.app/#load:https://i.imgur.com/zJMrWwb.png
  • Rudimentary multi-user collaboration support. Start up a session at jspaint.app/#session:multi-user-test and send the link to your friends! It isn't seamless; actions by other users interrupt what you're doing, and visa versa. Sessions are not private, and you may lose your work at any time. If you want better collaboration support, follow the development of Mopaint.
• Extras > Themes to change the look of the app. Dark mode included.
• Eye Gaze Mode, for use with an eye tracker, head tracker, or other coarse input device, accessible from Extras > Eye Gaze Mode. With just a webcam, you can try it out with Enable Viacam (head tracker) or GazePointer (eye tracker).
• Speech Recognition Mode. Using your voice you can select tools and colors, pan the view ("scroll down and to the left", or "go southwest", etc.), explore the menus (but you can activate any menu item without opening the menus first), interact with windows (including scrolling the history view with "scroll up"/"scroll down" etc.), dictate text with the Text tool, and even tell the application to sketch things (for instance, "draw a house")
• Create an animated GIF from the current document history. Accessible from the Extras menu or with Ctrl+Shift+G. It's pretty nifty, you should try it out! You might want to limit the size of the image though.
• Load and save many different palette formats with Colors > Get Colors and Colors > Save Colors. (I made a library for this: AnyPalette.js.)
  • You can also drag and drop palette files into the app to load.

Editing Features:

• Use Alt+Mousewheel to zoom in and out
• Edit transparent images! To create a transparent image, go to Image > Attributes... and select Transparent, then OK, and then Image > Clear Image or use the Eraser tool. Images with any translucent pixels will open in Transparent mode.
• You can crop the image by making a selection while holding Ctrl
• Keyboard shortcuts for rotation: Ctrl+. and Ctrl+, (< and >)
• Rotate by any arbitrary angle in Image > Flip/Rotate
• In Image > Stretch/Skew, you can stretch more than 500% at once
• Zoom to an arbitrary scale in View > Zoom > Custom...
• Zoom to fit the canvas within the window with View > Zoom > Zoom To Window
• Non-contiguous fill: Replace a color in the entire image by holding Shift when using the fill tool

Miscellaneous Improvements:

• Vertical Color Box mode, accessible from Extras > Vertical Color Box
• You can use the Text tool at any zoom level (and it previews the exact pixels that will end up on the canvas).
• Spellcheck is available in the textbox if your browser supports it.
• Resize handles are easier to grab than in Windows 10's Paint.
• Omits some Thumbnail view bugs, like the selection showing in the wrong place.
• Unlimited undos/redos (as opposed to a measly 3 in Windows XP, or a measly 50 in Windows 7)
• Undo history is nonlinear, which means if you undo and do something other than redo, the redos aren't discarded. Instead, a new branch is created in the history tree. Jump to any point in history with Edit > History or Ctrl+Shift+Y
• Automatically keeps a backup of your image. Only one backup per image tho, which doesn't give you a lot of safety. Remember to save with File > Save or Ctrl+S! Manage backups with File > Manage Storage.

Limitations:

A few things with the tools aren't done yet. See TODO.md

Full clipboard support in the web app requires a browser supporting the Async Clipboard API w/ Images, namely Chrome 76+ at the time of writing.

In other browsers you can still copy with Ctrl+C, cut with Ctrl+X, and paste with Ctrl+V, but data copied from JS Paint can only be pasted into other instances of JS Paint. External images can be pasted in.

Supported File Formats

Image Formats

⚠️ Saving as JPEG will introduce artifacts that cause problems when using the Fill tool or transparent selections.

⚠️ Saving in some formats will reduce the number of colors in the image.

💡 Unlike in MS Paint, you can use Edit > Undo to revert color or quality reduction from saving. This doesn't undo saving the file, but allows you to then save in a different format with higher quality, using File > Save As.

💡 Saving as PNG is recommended as it gives small file sizes while retaining full quality.

File Extension	Name	Read	Write	Read Palette	Write Palette
.png	PNG	✅	✅	🔜
.bmp, .dib	Monochrome Bitmap	✅	✅	🔜	✅
.bmp, .dib	16 Color Bitmap	✅	✅	🔜	✅
.bmp, .dib	256 Color Bitmap	✅	✅	🔜	✅
.bmp, .dib	24-bit Bitmap	✅	✅	N/A	N/A
.tif, .tiff, .dng, .cr2, .nef	TIFF (loads first page)	✅	✅
.pdf	PDF (loads first page)	✅
.webp	WebP	🌐	🌐
.gif	GIF	🌐	🌐
.jpeg, .jpg	JPEG	🌐	🌐	N/A	N/A
.svg	SVG (only default size)	🌐
.ico	ICO (only default size)	🌐

Capabilities marked with 🌐 are currently left up to the browser to support or not. If "Write" is marked with 🌐, the format will appear in the file type dropdown but may not work when you try to save. For opening files, see Wikipedia's browser image format support table for more information.

Capabilities marked with 🔜 may be coming soon, and N/A means not applicable.

"Read Palette" refers to loading the colors into the Colors box automatically (from an indexed color image), and "Write Palette" refers to writing an indexed color image.

Color Palette Formats

With Colors > Save Colors and Colors > Get Colors you can save and load colors in many different formats, for compatibility with a wide range of programs.

If you want to add extensive palette support to another application, I've made this functionality available as a library: AnyPalette.js

File Extension	Name	Programs	Read	Write
.pal	RIFF Palette	MS Paint for Windows 95 and Windows NT 4.0	✅	✅
.gpl	GIMP Palette	Gimp, Inkscape, Krita, KolourPaint, Scribus, CinePaint, MyPaint	✅	✅
.aco	Adobe Color Swatch	Adobe Photoshop	✅	✅
.ase	Adobe Swatch Exchange	Adobe Photoshop, InDesign, and Illustrator	✅	✅
.txt	Paint.NET Palette	Paint.NET	✅	✅
.act	Adobe Color Table	Adobe Photoshop and Illustrator	✅	✅
.pal, .psppalette	Paint Shop Pro Palette	Paint Shop Pro (Jasc Software / Corel)	✅	✅
.hpl	Homesite Palette	Allaire Homesite / Macromedia ColdFusion	✅	✅
.cs	ColorSchemer	ColorSchemer Studio	✅
.pal	StarCraft Palette	StarCraft	✅	✅
.wpe	StarCraft Terrain Palette	StarCraft	✅	✅
.sketchpalette	Sketch Palette	Sketch	✅	✅
.spl	Skencil Palette	Skencil (formerly called Sketch)	✅	✅
.soc	StarOffice Colors	StarOffice, OpenOffice, LibreOffice	✅	✅
.colors	KolourPaint Color Collection	KolourPaint	✅	✅
.colors	Plasma Desktop Color Scheme	KDE Plasma Desktop	✅
.theme	Windows Theme	Windows Desktop	✅
.themepack	Windows Theme	Windows Desktop	✅
.css, .scss, .styl	Cascading StyleSheets	Web browsers / web pages	✅	✅
.html, .svg, .js	any text files with CSS colors	Web browsers / web pages	✅

Did you know?

• There's a black and white mode with patterns instead of colors in the palette, which you can get to from Image > Attributes...

• You can drag the color box and tool box around if you grab them by the right place. You can even drag them out into little windows. You can dock the windows back to the side by double-clicking on their title bars.

• In addition to the left-click foreground color and the right-click background color, there's a third color you can access by holding Ctrl while you draw. It starts out with no color so you'll need to hold Ctrl and select a color first. The fancy thing about this color slot is you can press and release Ctrl to switch colors while drawing.

• You can apply image transformations like Flip/Rotate, Stretch/Skew or Invert (in the Image menu) either to the whole image or to a selection. Try scribbling with the Free-Form Select tool and then doing Image > Invert

• These Tips and Tricks from a tutorial for MS Paint also work in JS Paint:

  • Brush Scaling (+ & - on the number pad to adjust brush size)
  • "Custom Brushes" (hold Shift and drag the selection to smear it)
  • The 'Stamp' "Tool" (hold Shift and click the selection to stamp it)
  • Image Scaling (+ & - on the number pad to scale the selection by factors of 2)
  • Color Replacement (right mouse button with Eraser to selectively replace the foreground color with the background color)
  • The Grid (Ctrl+G & Zoom to 4x+)
  • Quick Undo (Pressing a second mouse button cancels the action you were performing. I also made it redoable, in case you do it by accident!)
  • Scroll Wheel Bug (Hmm, let's maybe not recreate this?)

Desktop App

PWA

JS Paint can be installed as a Progressive Web App (PWA), although it doesn't work offline yet. Look for the install prompt in the address bar.

PWA features:

• No address bar; middle-ground between web and native
• Cross-platform (macOS, Windows, Linux, Android, iOS)
• Basic file integration:
  • File > Open
  • File > Save downloads the file after asking for a filename and format
  • Drag and drop files onto the window to open them

Missing features:

• Directly saving to files is implemented but not enabled currently.

  I was concerned about data loss for two reasons: 1. the change in behavior of File > Save / Ctrl+S from effectively acting as Save As to overwriting files directly, although I made a warning dialog for this, with a don't show again option; 2. there was a bad bug with saved files ending up completely empty (zero bytes), which I don't know if was a bug in my code or in Chrome.
• Offline support is not implemented.

  I've taken a few stabs at this, and I'm not the only one, but there are some huge caveats, such as the development server not being able to live-reload without disabling the service worker.

Electron

I've also built it into a desktop app with Electron and Electron Forge. You can download it from the releases page.

Electron app features:

• Native-like experience (runs in a window with no address bar)
• Cross-platform (macOS, Windows, Linux)
• Clipboard support
• Files can be opened in various ways:
  • File > Open
  • Drag and drop onto window
  • Drag and drop onto dock icon on macOS
  • Drag and drop onto desktop shortcut
  • Right Click > Open With in file manager (macOS and Linux)
    • On Windows, you can manually paste the path to the executable into the Open With dialog, which you can find by right clicking the app in the taskbar, then right clicking the app's name, and selecting Properties. In the Shortcut tab, the Target field is the path to the executable. Once you open the app in this way, the app will show up in the Open With list, and if you select "Always", it will become the default app for that file type.
  • Command line: type jspaint path/to/file.png in the terminal
• File > Save will save directly to the file
• File > Set As Wallpaper (Tiled) and File > Set As Wallpaper (Centered)
• On macOS, an icon representing the currently open file is shown in the titlebar. You can drag this icon into other applications, for example to include the image you're editing in an email. The icon is dimmed while there are unsaved changes.

Electron app limitations

• Basics:
  • Execution is blocked by default on Mac and Windows
    • On macOS you need to Ctrl+click the file and then say Open
    • On Windows, you need to say "More info" and then "Run" (or "Run anyway"?)
    • I would need to pay a fee for code signing to avoid this. It's basically security by extortion.
  • There are no automatic updates. Apparently I would need to pay a fee for code signing to get this free service.
    • That said, Help > About Paint can tell you if JS Paint is out of date, at least in terms of news updates.
  • Electron is out of date. It may, for instance, contain image decoding vulnerabilities that have since been fixed. However, I've taken precautions to sandbox the app and restrict write access to a list of files explicitly opened in the app, the list being controlled by the main process, separate from the renderer process which would handle image decoding.
  • Only a single editor window can be opened at once.
  • The File menu's recent files list is not implemented, nor are OS-specific jump menus.
• Minor details:
  • A very confusing message is shown if you edit a document before clicking an Open link in the Manage Storage dialog.
  • WebGL error messages tell you to refresh without offering a way to reload; also, calling the app a web page feels unpolished
  • The File > Open dialog does not have an All Files (*.*) option, and the list of file types supported is not exhaustive; for example, AVIF images can be loaded but only by drag and drop
  • Drag and drop shows two "Save changes to X?" dialogs on top of each other?
• I'm not sure if all of these are still issues, need to retest them:
  • Quit doesn't exit app completely, only closes the window if it's open... intended behavior? shouldn't right click > Quit really quit? https://stackoverflow.com/questions/44316306/how-to-quit-electron-app-on-mac
  • Quit doesn't show/focus window when save changes prompt is shown on maybe mac/linux
  • Ctrl+C doesn't exit on mac/linux electron/electron#5273
    • this is because of editor_window.on("close") calling preventDefault and may be a feature but needs to show/focus the window
    • https://stackoverflow.com/questions/75362687/electron-js-processes-do-not-exit-on-app-quit
  • Opening an SVG file also isn't working via command line argument (dragging onto the shortcut in File Explorer) even though dragging and dropping into the window works.
    • Seems to load load SVG as a palette... Is this what I was running into?

Development Setup

Clone the repo.

Install Node.js if you don't have it, then open up a command prompt / terminal in the project directory.

Quality Assurance

Run npm run lint to check for spelling errors, type errors, code style issues, and other problems.

Run npm run format to automatically fix formatting issues, or npx eslint --fix to fix all auto-fixable issues.

The formatting rules are configured for compatibility with VS Code's built-in formatter.

Run npm test to run browser-based tests with Cypress. (It's slow to start up and run tests, unfortunately.)

Run npm run accept to accept any visual changes. This unfortunately re-runs all the tests, rather than accepting results of the previous test, so you could end up with different results than the previous test. If you use GitHub Desktop, you can view diffs of images, in four different modes.

To open the Cypress UI, first run npm run test:start-server, then concurrently npm run cy:open

Tests are also run in continuous integration with Travis CI.

Web App (https://jspaint.app)

After you've installed dependencies with npm i, use npm run dev to start a live-reloading server.

Make sure any layout-important styles go in layout.css. When updating layout.css, a right-to-left version of the stylesheet is generated, using RTLCSS.
You should test the RTL layout by changing the language to Arabic or Hebrew. Go to Extras > Language > العربية or עברית.
See Control Directives for how to control the RTL layout.

There is a VS Code launch task for attaching to Chrome for debugging. See .vscode/launch.json for usage instructions.

Desktop App (Electron)

• Install dependencies with npm i
• Start the electron app with npm run electron:start

electron-debug is included, so you can use F5/Ctrl+R to reload and F12/Ctrl+Shift+I to open the devtools.

You can build for production with npm run electron:make

There is a VS Code launch task for debugging the Electron main process. For the renderer process, you can use the embedded Chrome DevTools.

Deployment

JS Paint can be deployed using a regular web server.

Nothing needs to be compiled.

CORS proxy

Optionally, you can set up a CORS Anywhere server, for loading images from the web, if you paste a URL into JS Paint, or use the #load:<URL> feature with images that are not on the same domain.

By default it will use a CORS Anywhere instance set up to work with jspaint.app.

It is hosted for free on Heroku, and you can set up your own instance and configure it to work with your own domain.

You'll have to find and replace https://jspaint-cors-proxy.herokuapp.com with your own instance URL.

Multiplayer Support

Multiplayer support currently relies on Firebase, which is not open source software.

You could create a Firebase Realtime Database instance and edit JS Paint's sessions.js to point to it, replacing the config passed to initializeApp with the config from the Firebase Console when you set up a Web App.

But the multiplayer mode is very shoddy so far. It should be replaced with something open source, more secure, more efficient, and more robust.

Embed in your website

Simple

Add this to your HTML:

<iframe src="https://jspaint.app" width="100%" height="100%"></iframe>

Start with an image

You can have it load an image from a URL by adding #load:<URL> to the URL.

<iframe src="https://jspaint.app#load:https://jspaint.app/favicon.ico" width="100%" height="100%"></iframe>

Advanced

If you want to control JS Paint, how it saves/loads files, or access the canvas directly, there is an unstable API.

First you need to clone the repo, so you can point an iframe to your local copy.

The local copy of JS Paint has to be hosted on the same web server as the containing page, or more specifically, it has to share the same origin.

Having a local copy also means things won't break any time the API changes.

If JS Paint is cloned to a folder called jspaint, which lives in the same folder as the page you want to embed it in, you can use this:

<iframe src="jspaint/index.html" id="jspaint-iframe" width="100%" height="100%"></iframe>

If it lives somewhere else, you may need to add ../ to the start of the path, to go up a level. For example, src="../../apps/jspaint/index.html". You can also use an absolute URL, like src="https://example.com/cool-apps/jspaint/index.html".

Changing how files are saved/loaded

You can override the file saving and opening dialogs with JS Paint's systemHooks API.

<script>
var iframe = document.getElementById("jspaint-iframe");
var jspaint = iframe.contentWindow;
// Wait for systemHooks object to exist (the iframe needs to load)
waitUntil(()=> jspaint.systemHooks, 500, ()=> {
	// Hook in
	jspaint.systemHooks.showSaveFileDialog = async ({ formats, defaultFileName, defaultPath, defaultFileFormatID, getBlob, savedCallbackUnreliable, dialogTitle }) => { ... };
	jspaint.systemHooks.showOpenFileDialog = async ({ formats }) => { ... };
	jspaint.systemHooks.writeBlobToHandle = async (save_file_handle, blob) => { ... };
	jspaint.systemHooks.readBlobFromHandle = async (file_handle) => { ... };
});
// General function to wait for a condition to be met, checking at regular intervals
function waitUntil(test, interval, callback) {
	if (test()) {
		callback();
	} else {
		setTimeout(waitUntil, interval, test, interval, callback);
	}
}
</script>

A Blob represents the contents of a file in memory.

A file handle is anything that can identify a file. You get to own this concept, and define how to identify files. It could be anything from an index into an array, to a Dropbox file ID, to an IPFS URL, to a file path. It can be any type, or maybe it needs to be a string, I forget.

Once you have a concept of a file handle, you can implement file pickers using the system hooks, and functions to read and write files.

Command	Hooks Used
File > Save As	systemHooks.showSaveFileDialog, then when a file is picked, systemHooks.writeBlobToHandle
File > Open	systemHooks.showOpenFileDialog, then when a file is picked, systemHooks.readBlobFromHandle
File > Save	systemHooks.writeBlobToHandle (or same as File > Save As if there's no file open yet)
Edit > Copy To	systemHooks.showSaveFileDialog, then when a file is picked, systemHooks.writeBlobToHandle
Edit > Paste From	systemHooks.showOpenFileDialog, then when a file is picked, systemHooks.readBlobFromHandle
File > Set As Wallpaper (Tiled)	systemHooks.setWallpaperTiled if defined, else systemHooks.setWallpaperCentered if defined, else same as File > Save As
File > Set As Wallpaper (Centered)	systemHooks.setWallpaperCentered if defined, else same as File > Save As
Extras > Render History As GIF	Same as File > Save As
Colors > Save Colors	Same as File > Save As
Colors > Get Colors	Same as File > Open

Loading a file initially

To start the app with a file loaded for editing, wait for the app to load, then call systemHooks.readBlobFromHandle with a file handle, and tell the app to load that file blob.

const file_handle = "initial-file-to-load";
systemHooks.readBlobFromHandle(file_handle).then(file => {
	if (file) {
		contentWindow.open_from_file(file, file_handle);
	}
}, (error) => {
	// Note: in some cases, this handler may not be called, and instead an error message is shown by readBlobFromHandle directly.
	contentWindow.show_error_message(`Failed to open file ${file_handle}`, error);
});

This is clumsy, and in the future there may be a query string parameter to load an initial file by its handle. (Note to self: it will need to wait for your system hooks to be registered, somehow.)

There's already a query string parameter to load from a URL:

<iframe src="https://jspaint.app?load:SOME_URL_HERE"></iframe>

But this won't set up the file handle for saving.

Integrating Set as Wallpaper

You can define two functions to set the wallpaper, which will be used by File > Set As Wallpaper (Tiled) and File > Set As Wallpaper (Centered).

• systemHooks.setWallpaperTiled = (canvas) => { ... };
• systemHooks.setWallpaperCentered = (canvas) => { ... };

If you define only systemHooks.setWallpaperCentered, JS Paint will attempt to guess your screen's dimensions and tile the image, applying it by calling your systemHooks.setWallpaperCentered function.

If you don't specify systemHooks.setWallpaperCentered, JS Paint will default to saving a file (<original file name> wallpaper.png) using systemHooks.showSaveFileDialog and systemHooks.writeBlobToHandle.

Here's a full example supporting a persistent custom wallpaper as a background on the containing page:

const wallpaper = document.querySelector("body"); // or some other element

jspaint.systemHooks.setWallpaperCentered = (canvas) => {
	canvas.toBlob((blob) => {
		setDesktopWallpaper(blob, "no-repeat", true);
	});
};
jspaint.systemHooks.setWallpaperTiled = (canvas) => {
	canvas.toBlob((blob) => {
		setDesktopWallpaper(blob, "repeat", true);
	});
};

function setDesktopWallpaper(file, repeat, saveToLocalStorage) {
	const blob_url = URL.createObjectURL(file);
	wallpaper.style.backgroundImage = `url(${blob_url})`;
	wallpaper.style.backgroundRepeat = repeat;
	wallpaper.style.backgroundPosition = "center";
	wallpaper.style.backgroundSize = "auto";
	if (saveToLocalStorage) {
		const fileReader = new FileReader();
		fileReader.onload = () => {
			localStorage.setItem("wallpaper-data-url", fileReader.result);
			localStorage.setItem("wallpaper-repeat", repeat);
		};
		fileReader.onerror = () => {
			console.error("Error reading file (for setting wallpaper)", file);
		};
		fileReader.readAsDataURL(file);
	}
}

// Initialize the wallpaper from localStorage, if it exists
try {
	const wallpaper_data_url = localStorage.getItem("wallpaper-data-url");
	const wallpaper_repeat = localStorage.getItem("wallpaper-repeat");
	if (wallpaper_data_url) {
		fetch(wallpaper_data_url).then(response => response.blob()).then(file => {
			setDesktopWallpaper(file, wallpaper_repeat, false);
		});
	}
} catch (error) {
	console.error(error);
}

It's a little bit recursive, sorry; it could probably be done simpler. Like by just using data URLs. (Actually, I think I wanted to use blob URLs just so that it doesn't bloat the DOM inspector with a super long URL. Which is really a devtools UX bug. Maybe they've improved this?)

Specifying the canvas size

You can load a file that has the desired dimensions. There's no special API for this at the moment.

See Loading a file initially.

Specifying the theme

You could change the theme programmatically:

var iframe = document.getElementById("jspaint-iframe");
var jspaint = iframe.contentWindow;
jspaint.set_theme("modern.css");

but this will break the user preference.

The Extras > Themes menu will still work, but the preference won't persist when reloading the page.

In the future there may be a query string parameter to specify the default theme. You could also fork jspaint to change the default theme.

Specifying the language

Similar to the theme, you can try to change the language programmatically:

var iframe = document.getElementById("jspaint-iframe");
var jspaint = iframe.contentWindow;
jspaint.set_language("ar");

but this will actually ask the user to reload the application to change languages.

The Extras > Language menu will still work, but the user will be bothered to change the language every time they reload the page.

In the future there may be a query string parameter to specify the default language. You could also fork jspaint to change the default language.

Adding custom menus

Not supported yet. You could fork jspaint and add your own menus.

Accessing the canvas directly

With access to the canvas, you can implement a live preview of your drawing, for example updating a texture in a game engine in realtime.

var iframe = document.getElementById("jspaint-iframe");
// contentDocument here refers to the webpage loaded in the iframe, not the image document loaded in jspaint.
// We're just reaching inside the iframe to get the canvas.
var canvas = iframe.contentDocument.querySelector(".main-canvas");

It's recommended not to use this for loading a document, as it won't change the document title, or reset undo/redo history, among other things. Instead use open_from_file.

Performing custom actions

If you want to make buttons or other UI to do things to the document, you should (probably) make it undoable. It's very easy, just wrap your action in a call to undoable.

var iframe = document.getElementById("jspaint-iframe");
var jspaint = iframe.contentWindow;
var icon = new Image();
icon.src = "some-folder/some-image-15x11-pixels.png";
jspaint.undoable({
	name: "Seam Carve",
	icon: icon, // optional
}, function() {
	// do something to the canvas
});

async function systemHooks.showSaveFileDialog({ formats, defaultFileName, defaultPath, defaultFileFormatID, getBlob, savedCallbackUnreliable, dialogTitle })

Define this function to override the default save dialog. This is used both for saving images, as well as palette files, and animations.

Arguments:

• formats: an array of objects representing types of files, with the following properties:
  • formatID: a string that uniquely identifies the format (may be the same as mimeType)
  • mimeType (optional): the file format's designated media type, e.g. "image/png" (palette formats do not have this property)
  • name: the file format's name, e.g. "WebP"
  • nameWithExtensions: the file format's name followed by a list of extensions, e.g. "TIFF (*.tif;*.tiff)"
  • extensions: an array of file extensions, excluding the dot, with the preferred extension first, e.g. ["bmp", "dib"]
• defaultFileName (optional): a suggested file name, e.g. "Untitled.png" or the name of an open document.
• defaultPath (optional): a file handle for a document that was opened, so you can save to the same folder easily. Misnomer: this may not be a path, it depends on how you define file handles.
• defaultFileFormatID (optional): the formatID of a file format to select by default.
• async function getBlob(formatID): a function you call to get a file in one of the supported formats. It takes a formatID and returns a Promise that resolves with a Blob representing the file contents to save.
• function savedCallbackUnreliable({ newFileName, newFileFormatID, newFileHandle, newBlob }) (optional): a function you call when the user has saved the file. The newBlob should come from getBlob(newFileFormatID).
• dialogTitle (optional): a title for the save dialog.

Note the inversion of control here: JS Paint calls your systemHooks.showSaveFileDialog function, and then you call JS Paint's getBlob function. Once getBlob resolves, you can call the savedCallbackUnreliable function which is defined by JS Paint. (Hopefully I can clarify this in the future.)

Also note that this function is responsible for saving the file, not just picking a save location. You may reuse your systemHooks.writeBlobToHandle function if it's helpful.

async function systemHooks.showOpenFileDialog({ formats })

Define this function to override the default open dialog. This is used for opening images and palettes.

Arguments:

• formats: same as systemHooks.showSaveFileDialog

Note that this function is responsible for loading the contents of the file, not just picking a file. You may reuse your systemHooks.readBlobFromHandle function if it's helpful.

async function systemHooks.writeBlobToHandle(fileHandle, blob)

Define this function to tell JS Paint how to save a file.

Arguments:

• fileHandle: a file handle, as defined by your system, representing the file to write to.
• blob: a Blob representing the file contents to save.

Returns:

• Promise that resolves with true if the file was definitely saved successfully, false if an error occurred or the user canceled, or undefined if it is not known whether the file was saved successfully, as is the case with file downloading with <a href="..." download="...">. The promise should not reject; errors should be handled by showing an error message and returning false.

async function systemHooks.readBlobFromHandle(fileHandle)

Define this function to tell JS Paint how to load a file.

Arguments:

• fileHandle: a file handle, as defined by your system, representing the file to read from.

function systemHooks.setWallpaperTiled(canvas)

Define this function to tell JS Paint how to set the wallpaper. See Integrating Set as Wallpaper for an example.

Arguments:

• canvas: a HTMLCanvasElement with the image to set as the wallpaper.

function systemHooks.setWallpaperCentered(canvas)

Define this function to tell JS Paint how to set the wallpaper. See Integrating Set as Wallpaper for an example.

Arguments:

• canvas: a HTMLCanvasElement with the image to set as the wallpaper.

function undoable({ name, icon }, actionFunction)

Use this to make an action undoable.

This function takes a snapshot of the canvas, and some other state, and then calls the actionFunction function. It creates an entry in the history so it can be undone.

Arguments:

• name: a name for the action, e.g. "Brush" or "Rotate Image 270°"
• icon (optional): an Image to display in the History window. It is recommended to be 15x11 pixels.
• actionFunction: a function that takes no arguments, and modifies the canvas.

function show_error_message(message, [error])

Use this to show an error message dialog box, optionally with expandable error details.

Arguments:

• message: plain text to show in the dialog box.
• error (optional): an Error object to show in the dialog box, collapsed by default in a "Details" expandable section.

function open_from_file(blob, source_file_handle)

Use this to load a file into the app.

Arguments:

• blob: a Blob object representing the file to load.
• source_file_handle: a corresponding file handle for the file, as defined by your system.

Sorry for the quirky API. The API is new, and parts of it have not been designed at all. This was just a hack that I came to depend on, reaching into the internals of JS Paint to load a file. I decided to document it as the first version of the API, since I'll want a changelog when upgrading my usage of it anyways.

function set_theme(theme_file_name)

Use this to change the look of the application.

Arguments:

• theme_file_name: the name of the theme file to load, one of:
  • "classic.css": the Windows98 theme.
  • "dark.css": the Dark theme.
  • "modern.css": the Modern theme.
  • "winter.css": the festive Winter theme.
  • "occult.css": a Satanic theme.

function set_language(language_code)

You can kind of use this to change the language of the application. But actually it will show a prompt to the user to change the language, because the application needs to reload to apply the change. And if that dialog isn't in the right language, well, they'll probably be confused.

Arguments:

• language_code: the language code to use, e.g. "en" for English, "zh" for Traditional Chinese, "zh-simplified" for Simplified Chinese, etc.

Changelog

The API will change a lot, but changes will be documented in the Changelog.

Not just a history of changes, but a migration/upgrading guide.

For general project news, click Extras > Project News in the app.

License

JS Paint is free and open source software, licensed under the permissive MIT license.