doGet(e) param in GAS

// how to send parameters on GAS server using doGet(e)
//
// get the two parameters 'a' and 'b' and display the values
// Usage: https://...?a=1&b=2
//
// https://developers.google.com/apps-script/content_service
// http://ochi-lab.blogspot.jp/
//
function doGet(e) {
  
  var a = e.parameters.a;
  var b = e.parameters.b;
  var str = 'a = ' + a + ', b = ' + b;
  
  var output = ContentService.createTextOutput();
  output.setMimeType(ContentService.MimeType.JSON);
  output.setContent(str);
  return output;  
}