runner.js

function runner(gen, ...args) {
  // Initialize the generator.
  let iterator = gen(...args);    
  
  // handleNext is responsible for restarting the generator with new input.
  return Promise.resolve().then(function handleNext(value) {
    // next can be a promise or a sync value.
    let next = iterator.next(value);
    
    // handleResult is responsible for taking the output of the generator
    // and scheduling handleNext to run once it resolves.
    return (function handleResult(next) {
      if (next.done)
        return next.value;
      
      return Promise.resolve(next.value).then(
        handleNext,
        function error(err) {
          return Promise.resolve(iterator.throw(err)).then(handleResult);
      });
    })(next);
  });
}