clientside_image_download.js

/**
 * [downloadImage]
 * @param  {[string]} img       [base64encoded image data]
 * @param  {[string]} fileName  [new file name]
 * @return [image file]
 */
function downloadImage(img, fileName) {
  var imgData = atob(img.src.split(',')[1]),
    len = imgData.length,
    buf = new ArrayBuffer(len),
    view = new Uint8Array(buf),
    blob, 
    i;
  
  for (i = 0; i < len; i++) {
    view[i] = imgData.charCodeAt(i) & 0xff; // masking
  }

  blob = new Blob([view], {
    type: 'application/octet-stream'
  });
  
  if (window.navigator.msSaveOrOpenBlob) {
    window.navigator.msSaveOrOpenBlob(blob, fileName);
  } else {
    //var url = URL.createObjectURL(blob);
    var a = document.createElement('a');
    a.style = "display: none";
    //a.href = url;
    a.href = img.src;
    a.download = fileName;
    document.body.appendChild(a);
    a.click();
    
    setTimeout(function() {
      document.body.removeChild(a);
      //URL.revokeObjectURL(url);
    }, 100);
  }
}