Requires jszip and FileSaver

Download URLs to a zip file, without saving them to local.js

var urls = [
  './song.ini',
  './notes.chart',
  './album.png',
  './ch.dat',
  './song.ogg',
];
var images = [];
var counter = 0;
// From http://stackoverflow.com/questions/6150289/how-to-convert-image-into-base64-string-using-javascript

async function getFileFromUrl(url, callback){
  const response = await fetch(url);
  const data = await response.blob();
  var req = new XMLHttpRequest();
	req.open('HEAD', url);

	req.onreadystatechange = function() {
		var header = req.getResponseHeader('Content-Disposition');
		var filename = header.match(/filename="(.+)"/)[1]; // image.jpg
		console.log(filename);
		blobToBase64(data, function (dataURL) { callback(dataURL, url, filename); })
	};
  req.send(null);
  console.log(data);
  
}

function createArchive(images){
  // Use jszip
  var zip = new JSZip();
  var img = zip.folder("images");
  
  for (var i=0; i<images.length; i++) {
    var commaIdx = images[i].data.indexOf(",");
	img.file(images[i].filename, images[i].data.slice(commaIdx + 1), {base64: true});
  }
  zip.generateAsync({type:"blob"}).then(function(file){
	saveAs(file, "images.zip");
  })
}

var blobToBase64 = function(blob, callback) {
    var reader = new FileReader();
    reader.onload = function() {
        var dataUrl = reader.result;
        var base64 = dataUrl.split(',')[1];
        callback(base64);
    };
    reader.readAsDataURL(blob);
};

for (var i = 0; i<urls.length; i++) {
  getFileFromUrl(urls[i], function (base64Img, url, filename) {
    images.push({
      url: url,
      data: base64Img,
	  filename: filename,
    });
    counter++;
	console.log("counter" + counter);
	console.log(images);
	console.log(urls.length);
    if (counter >= (urls.length * 2)) {
      createArchive(images);
    }
  });
}