offscreen_worker2.js

let canvas;
let ctx;

function animate() {
  // do  animations here ...

  const bitmap = canvas.transferToImageBitmap();
  self.postMessage({msg: 'render', bitmap}); // this will be handled in main thread , where we will update the content 
  self.requestAnimationFrame(animate);
}

self.onmessage = function(ev) {
  if(ev.data.msg === 'offscreen') {
    canvas = ev.data.canvas;
    ctx = canvas.getContext('2d');
    animate();
  }
}