spawn-example.js

/**
 * Create a `spawn` function to deal with promises
 * http://blog.mgechev.com/2014/12/21/handling-asynchronous-calls-with-es6-javascript-generators/
 */
function spawn(gen) {
  const it = gen(); //instantiate the generator and return the generator object

  function _co(method, arg) {
    let res;

    try {
      //retrieve the promise returned by the http request if `arg` is undefined
      //if `arg` is defined it will be the data from the http request promise
      //and will be "injected" into `yield` and caught in a variable
      res = it[method](arg);
    } catch(err) {
      //not sure about the error handling here??
      return Promise.reject(err);
    }

    if (res.done) {
      if (method === 'throw') {
        return arg;
      } else {
        return res.value;
      }
    } else {
      //at this point we may resolve a promise or a value??
      //a) if we are resolving a promise we will inject it's value by calling `.next`
      //b) if we are resolving a value it will be ignored by `.next` as `yield` will be
      //   the returned Promise from the http request
      return Promise.resolve(res.value)
        .then((val) => {
          return _co('next', val);
        }, (err) => {
          //not sure about the error handling here??
          return _co('throw', err);
        });
    }
  }

  return _co('next'); //start the process by calling `.next` on the generator instance
}