A small library to make it easier to use OffscreenCanvas in a Web Worker

Usage no npm install needed!

<script type="module">
  import ezOffscreenCanvas from 'https://cdn.skypack.dev/ez-offscreen-canvas';



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This is a tiny library to make it easier to use OffscreenCanvas in a Web Worker. In particular, it does the following:

  1. Creates the OffscreenCanvas instance from a given <canvas> element.
  2. Passes the OffscreenCanvas and other values from the main thread to a Worker thread.
  3. Allows you to write your Worker code in the main thread as a function.
  4. Falls back to running in the main thread if OffscreenCanvas is not supported.

Here's a short example of how it is used:


<script type="module">
   import { ezOffscreenCanvas } from 'https://unpkg.com/ez-offscreen-canvas';

  const canvas = document.querySelector('canvas');
  const props = { color: 'red' };

  // The code in this function will run in a Web Worker
  // which means it can't access variables in the closure
  const render = ({ canvas, color }) => {
    const context = canvas.getContext('2d');

    const draw = () => {
      context.fillStyle = color;
      context.fillRect(0, 0, canvas.width, canvas.height);



  const offscreenCanvas = ezOffscreenCanvas(canvas, props, render);

  // Since control has been transferred to an OffscreenCanvas
  // we can't set attributes on the original canvas directly
  offscreenCanvas.setAttributes({ width: 700, height: 350 });

  // Make sure we cleanup the Web Worker when no longer needed
  setTimeout(offscreenCanvas.terminate, 10000);

See the above code running live on CodePen.

Requiring OffscreenCanvas in a Web Worker

If you only want the render function to run if OffscreenCanvas is available and can run in a Web Worker, then pass an options hash with workerOnly set to true as the fourth argument to the ezOffscreenCanvas method:

ezOffscreenCanvas(canvas, props, render, { workerOnly: true });