function toDataUri(img, scalar = 1) {

  const canvas = document.createElement('canvas');

  canvas.width = img.width * scalar;

  canvas.height = img.height * scalar;

  canvas.getContext('2d').drawImage(img, 0, 0, canvas.width, canvas.height);

  return canvas.toDataURL('image/png');

}

const img = new Image();

img.crossOrigin = 'Anonymous';

img.addEventListener('load', () => {

  const thumb = new Image();

  // use the data URI as the source

  thumb.src = toDataUri(img, .3);

  document.body.appendChild(thumb);

});

img.src = 'https://zevanrosser.com/fotoda/slide-shot-9.jpg';