function toDataURL(url, callback) {
  var xhr = new XMLHttpRequest();
  xhr.onload = function() {
    var reader = new FileReader();
    reader.onloadend = function() {
      callback(reader.result);
    }
    reader.readAsDataURL(xhr.response);
  };
  xhr.open('GET', url);
  xhr.responseType = 'blob';
  xhr.send();
}

toDataURL('https://cdn.discordapp.com/attachments/571092147801948204/784586541146177606/6f32c864-985a-481d-8d8e-bd1f14ab9951.png', function(dataUrl) {
  console.log('RESULT:', dataUrl)
})