Instead of:

  var result = '';
  get('a', function(response) {
    result += JSON.decode(response).data;
    get('b', function(response) {
      result += JSON.decode(response).data;
      get('c', function(response) {
        result += JSON.decode(response).data;
        get('d', function(response) {
          result += JSON.decode(response).data;
          get('e', function(response) {
            result += JSON.decode(response).data;
            get('f', function(response) {
              result += JSON.decode(response).data;
              console.log(result);
            });
          });
        });
      });
    });
  });

  function example()
  {
    var resources = ['a', 'b', 'c', 'd', 'e', 'f'];
    var result = "";
    while(resource = resources.shift())
    {
      result = add(result, get(resource));
    }
    show(result); // -> abcdef
  }