offscreencanvas2.js

const canvas = document.querySelector('#canvas');
const ctx = canvas.getContext('bitmaprenderer'); // 
const offscreenCanvas = new OffscreenCanvas(canvas.width, canvas.height);

const worker = new Worker('offscreen_worker2.js');
worker.postMessage({msg: 'init', canvas: offscreenCanvas}, [offscreenCanvas]);

worker.addEventListener('message', function(ev) {
  if(ev.data.msg === 'render') {
    ctx.transferFromImageBitmap(ev.data.bitmap);
  }
});