worker.js

importScripts('./node_modules/comlink/dist/umd/comlink.min.js');

async function resizer(file,offscreencanvas,max)
{
  let width = max,height = max;
  let img = await createImageBitmap(file);

  let ratio = img.width / img.height;
  if(ratio < 1 && img.width <= max)
    return file;

  if(ratio >= 1 && img.height <= max)
    return file;

  // calculate resized image width/height
  if(img.width > img.height)
    width = height * img.width / img.height;
  else if(img.width < img.height) 
    height = width * img.height / img.width;

  offscreencanvas.width = width;
  offscreencanvas.height = height;
  
  let ctx = offscreencanvas.getContext('2d');

  // clear canvas rect
  ctx.clearRect(0,0,width,height);

  // draw image to canvas
  ctx.drawImage(
    img,
    0,
    0,
    img.width,
    img.height,
    0,
    0,
    width,
    height
  );

  return new File([await getBlobFromCanvas(offscreencanvas,file.type)],file.name,{type: file.type,lastModified: file.lastModified});
}

function getBlobFromCanvas(offscreencanvas,type,quality)
{
  type = type || 'image/png';
  quality = quality || 0.8;

  return offscreencanvas.convertToBlob({'type': type,'quality': quality});
}

Comlink.expose(resizer);