Assigning `yield` results...

assigning-yield-results.js

const next = (iter, callback, prev = undefined) => {
  // 2. The yielded value is extracted by calling
  // .next(). We pass the previous value back into
  // the generator for assignment.
  const item = iter.next(prev);
  const value = item.value;

  // 4. The final value gets passed to the callback.
  if (item.done) return callback(prev);

  if (isPromise(value)) {
    value.then(val => {
      setImmediate(() => next(iter, callback, val));
    });
  } else {
    setImmediate(() => next(iter, callback, value));
  }
};

const asyncFunc = gensync(function* () {
  // 1. yield value gets passed to the iterator.
  // The function exits at the yield call time,
  // and the `result` assignment doesn't happen
  // until the generator is resumed.
  const result = yield fetchSomething();

  // 3. Does not run until .next() is called again.
  // `result` will contain the value passed into
  // the previous `.next()` call.
  yield result + ' 2';
});