index.js

// Thunk
let get = function (url) {

  // return a function, passing in our callback
  return function (callback) {
    let xhr = new XMLHttpRequest();
    xhr.open('GET', url);
    xhr.onreadystatechange = function() {
      let response = xhr.responseText;
      if(xhr.readyState != 4) return;
      if (xhr.status === 200) {
        callback(null, response);
      }
      else {
        callback(response, null);
      }
    };
    xhr.send();
  };
};


let getTweets = function* () {
  let totalTweets = [];
  let data;

  // pause and get the 1st tweet. Continue when next() is called
  data = yield get('https://api.myjson.com/bins/2qjdn');
  totalTweets.push(data);

  // pause and get the 2nd tweet. Continue when next() is called
  data = yield get('https://api.myjson.com/bins/3zjqz');
  totalTweets.push(data);

  // then get the 3rd tweet. Continue when next() is called
  data = yield get('https://api.myjson.com/bins/29e3f');
  totalTweets.push(data);

  return totalTweets;
};


var cb = function(err, res) {
  if (err) return console.error(err);
  console.log(res);
};

// iterator object
let iterator = getTweets();

iterator.next().value(cb);
iterator.next().value(cb);
iterator.next().value(cb);
iterator.next();