Control the flow of asynchronous calls with ES6 generator functions

index.js

/**
 * get - XHR Request
 */

let get = function (url) {
  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();
  };
};


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

  // get the 1st tweet
  data = yield get('https://api.myjson.com/bins/2qjdn');
  totalTweets.push(data);

  // now get the 2nd tweet
  data = yield get('https://api.myjson.com/bins/3zjqz');
  totalTweets.push(data);

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

  console.log(totalTweets);
}

/**
 * runGenerator
 * A function that takes a generator function and
 * recusively calls next() until `done: true`
 */
 
let runGenerator = function (fn) {

  let next = function (err, arg) {
    if (err) return it.throw(err);

    var result = it.next(arg);
    if (result.done) return;

    if (typeof result.value == 'function') {
      result.value(next);
    }
    else {
      next(null, result.value);
    }
  }

  let it = fn();
  return next();
}

// kick it off
runGenerator(getTweets);

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