async_as_gen.js

function _asyncToGenerator(fn) { 
    return function () { 
        var self = this;
        var args = arguments; 
        return new Promise(function (resolve, reject) { 
            // Instantiates the generator
            var gen = fn.apply(self, args); 
            function _next(value) {
                // Next step of the generator
                var info = gen.next(value); 
                var newValue = info.value; 
    
                if (info.done) { 
                    resolve(newValue); 
                } else {
                    newValue.then(_next); 
                }
            } 
            // Calls the generator recursively until it's done
            _next(undefined); 
        }); 
   }; 
}

function f1() {
   return Promise.resolve(21); 
}
function f2(x) {
    return Promise.resolve(x * 2); 
}
  
function g() {
    _g = _asyncToGenerator(function* () {
      r = yield f1();
      s = yield f2(r);
      return s;
    });
    return _g.apply(this, arguments);
}
  
g().then(r => console.log(r));