How to download a file using Capybara and Poltergeist in Ruby. Tutorial Link https://goo.gl/CvzTG9

base64encode.js

var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

function base64encode(str) {
  var out, i, len;
  var c1, c2, c3;

  len = str.length;
  i = 0;
  out = "";
  while(i < len) {
  c1 = str.charCodeAt(i++) & 0xff;
  if(i == len)
  {
      out += base64EncodeChars.charAt(c1 >> 2);
      out += base64EncodeChars.charAt((c1 & 0x3) << 4);
      out += "==";
      break;
  }
  c2 = str.charCodeAt(i++);
  if(i == len)
  {
      out += base64EncodeChars.charAt(c1 >> 2);
      out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));
      out += base64EncodeChars.charAt((c2 & 0xF) << 2);
      out += "=";
      break;
  }
  c3 = str.charCodeAt(i++);
  out += base64EncodeChars.charAt(c1 >> 2);
  out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));
  out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >>6));
  out += base64EncodeChars.charAt(c3 & 0x3F);
  }
  return out;
}

download.rb

def download_file(remote_file_url)
    result = spider.evaluate_script(<<-JAVASCRIPT) # spider -> Capybara::Session.new(:poltergeist)
      (function(){
        var file;
        var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

        function base64encode(str) {
          var out, i, len;
          var c1, c2, c3;
        
          len = str.length;
          i = 0;
          out = "";
          while(i < len) {
          c1 = str.charCodeAt(i++) & 0xff;
          if(i == len)
          {
              out += base64EncodeChars.charAt(c1 >> 2);
              out += base64EncodeChars.charAt((c1 & 0x3) << 4);
              out += "==";
              break;
          }
          c2 = str.charCodeAt(i++);
          if(i == len)
          {
              out += base64EncodeChars.charAt(c1 >> 2);
              out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));
              out += base64EncodeChars.charAt((c2 & 0xF) << 2);
              out += "=";
              break;
          }
          c3 = str.charCodeAt(i++);
          out += base64EncodeChars.charAt(c1 >> 2);
          out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));
          out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >>6));
          out += base64EncodeChars.charAt(c3 & 0x3F);
          }
          return out;
        }

        function getBinary(url){
            var xhr = new XMLHttpRequest();
            xhr.open("GET", url, false);
            xhr.overrideMimeType("text/plain; charset=x-user-defined");
            xhr.send(null);
            return xhr.responseText;
        }

        file = base64encode(getBinary("#{remote_file_url}"))
        return file;
      })();
    JAVASCRIPT

    download_path = "tmp/report.pdf" # in my case I was downloading pdf files

    File.open(path, 'wb') do |f|
      f.write(Base64.decode64(result)) # decode the base64-encoded contents using Ruby's Base64.decode64 method
    end
    return download_path # you can do whatever it is really with the freshly downloaded file
end

getBinary.js

function getBinary(url){
    var xhr = new XMLHttpRequest();
    xhr.open("GET", url, false);
    xhr.overrideMimeType("text/plain; charset=x-user-defined");
    xhr.send(null);
    return xhr.responseText;
}

sniff_file_url.rb

def sniff_file_url
  time = 1
  url  = nil
  loop do
    time+= 1
    print "."
    unless spider.driver.network_traffic.last.response_parts.last.nil?
      url = spider.driver.network_traffic.last.response_parts.last.url
      # Find something unique to identify the url you want. In my case the download url had the text 'queue_pdf' with
      # the ready parameter set to 1. I used a simple regex below to match that
      break if url.match(/queue_pdf\/.*?ready=1/) || time > 30
    end
    sleep time
  end
  url # return the url
end