Using generators and promises for more understandable async pipelines.

iterate-promises.js

// This demo uses Babel for es2015 compilation and Promises, and superagent for requests.

// An example async pipeline, using a demo API
document.getElementById('run').addEventListener('click', (evt) => {
  iteratePromises(function* () {
    log('Requesting list of posts');
    let postsResponse = yield superagent.get(
      'http://jsonplaceholder.typicode.com/posts/');
    
    log('Requesting author of first post');
    let userResponse = yield superagent.get(
      'http://jsonplaceholder.typicode.com/users/' + postsResponse.body[0].userId);
    
    log('Post Author: ' + userResponse.body.name);
  });
});

// This helper takes a generator and resolves each yielded promise in turn, passing
// each result back as the `yield` response.
function iteratePromises(Generate) {
  let generator = Generate(); // store the iterable generator object
  function iterate(yieldResponse) {
    let iterated = generator.next(yieldResponse); // fire the next iterable step
    if (!iterated.done && typeof iterated.value.then === 'function') {
      // if the iterable yields a promise, wait for it to resolve and iterate again
      iterated.value.then((response) => iterate(response));
    }
  }
  iterate(); // kick off the generator
}

// A simple reverse-chronological logger
const log = (function (){
  const logTarget = document.getElementById('log');
  let logNumber = 1;
  return function (msg) {
    console.log(msg);
    logTarget.innerHTML = logNumber++ + ' >> ' + msg + '\n' + logTarget.innerHTML;
  };
}());

iteratePromises is a very simple implementation of Bluebird.coroutine, allowing async pipelines to be written using yield in a generator function.