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I wanted to run CanvasKit WASM in Cloudflare Workers to generate graphics server-side. Here's what I figured out.

\n
Live Example: CanvasKit WASM Tic-Tac-Toe - An interactive game using CanvasKit WASM, Cloudflare Workers, and classic HTML image maps.
\n

\n
Complete Example: canvaskit-wasm-worker - Full implementation you can follow along with or use directly.
\n

The Problem

CanvasKit WASM doesn't work out of the box in Cloudflare Workers because:

No file system for WASM loading
Different WASM instantiation requirements
Font loading through Workers assets
Memory management in serverless

WASM Instantiation

The CanvasKit WASM docs, and in fact the typescript types, only show a locateFile property:

CanvasKitInit({\n    locateFile: (file) => '/node_modules/canvaskit-wasm/bin/' + file,\n})\n

We can't use locateFile in Cloudflare Workers, but if you look closely, you may recognize that this is actually an Emscripten initializer.

It secretly has some other properties too. For Cloudflare Workers, the key is the instantiateWasm callback:

import canvasKitWasm from 'canvaskit-wasm/bin/canvaskit.wasm' // WebAssembly.Module\n\nconst CanvasKit = await CanvasKitInit({\n    instantiateWasm(imports, callback) {\n        let instance = new WebAssembly.Instance(canvasKitWasm, imports)\n        callback(instance)\n        return instance.exports\n    },\n})\n

Also need to set __dirname to avoid errors:

globalThis.__dirname = undefined\n

CanvasKit WASM is compiled to target a Node.js environment, so it tries to access the global __dirname during initialization. Setting it to undefined prevents errors in Cloudflare Workers where this variable doesn't exist (even with Node.js compatibility mode enabled).

You may also encounter errors related to path or fs modules. If so, add Node.js compatibility mode to your wrangler.jsonc:

{\n    "compatibility_flags": ["nodejs_compat"],\n}\n

This enables Node.js APIs in Cloudflare Workers, which helps resolve module-related errors from CanvasKit's Node.js-targeted compilation.

Font Loading

Font assets can be loaded from anywhere. One convenient option is to load them through the Workers static asset system:

// wrangler.jsonc\n{\n    "assets": { "directory": "./public/", "binding": "ASSETS" },\n}\n

// worker.ts\nconst assetUrl = new URL('/roboto-latin-500-normal.woff', request.url)\nconst robotoTypeface = await env.ASSETS.fetch(assetUrl)\n    .then((response) => response.arrayBuffer())\n    .then((fontData) => CanvasKit.Typeface.MakeTypefaceFromData(fontData))\n

This would expose your font asset file to the public internet, unless you take extra steps to prevent that. But Cloudflare does not charge bandwidth for static assets and typically fonts don't contain private data so for most people I think this should be acceptable.

Drawing Example

Based on the Skia CanvasKit example:

async function createImage(CanvasKit: CanvasKit, roboto: Typeface) {\n    let surface = CanvasKit.MakeSurface(300, 300)\n    const canvas = surface.getCanvas()\n\n    const paint = new CanvasKit.Paint()\n    paint.setColor(CanvasKit.Color(66, 129, 164, 1.0))\n    paint.setStyle(CanvasKit.PaintStyle.Stroke)\n    paint.setStrokeWidth(5.0)\n\n    canvas.clear(CanvasKit.Color(255, 255, 255, 1.0))\n    canvas.drawText('Hello Workers!', 10, 280, textPaint, textFont)\n\n    surface.flush()\n    const img = surface.makeImageSnapshot()\n    const pngBytes = img.encodeToBytes()\n\n    // Clean up memory\n    paint.delete()\n    surface.dispose()\n\n    return pngBytes\n}\n

Memory Management

The CanvasKit docs say to delete objects to "free up memory in the C++ WASM memory block". I'm not exactly sure but it sounds like it may be easy to cause a memory leak.

dpe.delete()\nskpath.delete()\ntextPaint.delete()\npaint.delete()\ntextFont.delete()\nsurface.dispose()\n

I wonder if we could use JavaScript's new using declaration to handle this automatically:

function useResource<T extends { delete(): void }>(obj: T) {\n    obj[Symbol.dispose] = obj.delete\n    return obj\n}\n\n{\n    using paint = useResource(new CanvasKit.Paint())\n    // paint automatically deleted when scope ends\n}\n

But I haven't tested this approach yet.

Caching

The CanvasKitInit function itself seems to cost around 50ms of CPU time.

I'm not sure if this will even run on Cloudflare Worker free plans, which have a nominal limit of 10ms CPU time per request.

In any case, it seems wise to cache the result of running CanvasKitInit across requests:

let canvasKitPromise: Promise<CanvasKit> | null = null\n\nif (!canvasKitPromise) {\n    canvasKitPromise = CanvasKitInit({...})\n}\n

This brought down the CPU time on subsequent requests. This works because Cloudflare Workers reuse the same global scope across requests, so module-level variables persist between invocations.

Practicalities

Size and CPU Time

When I uploaded my example, it showed the following size report:

Total Upload: 7211.12 KiB / gzip: 2860.67 KiB\n

Workers Free supports workers up to 3MB in size after compression and 10MB in size on Workers Paid. Therefore, canvaskit-wasm should fit just barely on the free plan and with a decent amount of room on the paid plan.

The simple example script above took about 65ms of CPU time on cold start and 15ms on warm start. It's not so bad, all things considered. I imagine the time depends on the resolution, complexity and output file format (png, jpeg, webp).

Consider Alternatives

If you are writing some application for Cloudflare Workers, it would be good to evaluate if you can render on the client size in JavaScript, or perhaps return something like a svg response to the user for their browser to render.

If you really need to render a raster image on the server, it may be good to cache the response or avoid generating too many bespoke images.

Service Bindings Architecture

Given the relative heaviness of CanvasKit, it may be wise to separate the CanvasKit rendering logic into its own worker and use service bindings. This approach would have several benefits:

Isolation: CanvasKit's memory usage and CPU costs are contained in a dedicated worker
Reusability: Multiple workers can share the same rendering service
Cold start optimization: The rendering worker stays warm from multiple callers

Here's how you might structure this:

// Image renderer worker (image-renderer.ts)\nimport { WorkerEntrypoint } from 'cloudflare:workers'\n\nexport default class ImageRenderer extends WorkerEntrypoint {\n    async renderImage(\n        width: number,\n        height: number,\n        text: string\n    ): Promise<ArrayBuffer> {\n        const CanvasKit = await CanvasKitInit({\n            // ...\n        })\n        // ... CanvasKit rendering logic ...\n        const pngBytes = img.encodeToBytes()\n        return pngBytes.buffer\n    }\n}\n

Then consume it from your main worker:

// Main worker\nexport default {\n    async fetch(request, env) {\n        const result = await env.IMAGE_RENDERER.renderImage(\n            300,\n            300,\n            'Hello Workers!'\n        )\n        return new Response(result, {\n            headers: { 'content-type': 'image/png' },\n        })\n    },\n}\n

This pattern would keep your main application lightweight while offloading the heavy CanvasKit work to a specialized service.

Key Takeaways

Use instantiateWasm instead of locateFile for CanvasKit in Cloudflare Workers
Cache the CanvasKitInit result - 50ms initialization cost, but 15ms warm requests
Memory management - manually call .delete() on CanvasKit objects to prevent WASM memory leaks
Bundle size fits within limits - 2.9MB compressed, well under Workers' 10MB paid tier limit
Consider client-side rendering first - server-side generation costs 65ms cold start, use sparingly

Complete Example

See the full implementation at canvaskit-wasm-worker.

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Posts tagged "canvas"

CanvasKit WASM in Cloudflare Workers