对于下面这段 js

function async3() {
  createPromise()
    .then((res) => {
        console.log(res);
    })
    .catch((err) => {
        console.log(err);
    })
    .finally(() => {
        console.log('finally');
    });
}

其对应 async/await 版本是

async function async2() {
    try {
      const ret = await createPromise();
      console.log(ret);
    } catch (e) {
      console.error(e);
    } finally {
      console.log('finally');
    }
}

经 babel 转为 ES5 后，除了必要的 babel-polypill，会生成以下代码，其中 _asyncToGenerator 不会多次生成。

var async2 = function () {
  var _ref = _asyncToGenerator( /*#__PURE__*/regeneratorRuntime.mark(function _callee() {
    var ret;
    return regeneratorRuntime.wrap(function _callee$(_context) {
      while (1) {
        switch (_context.prev = _context.next) {
          case 0:
            _context.prev = 0;
            _context.next = 3;
            return createPromise();

          case 3:
            ret = _context.sent;

            console.log(ret);
            _context.next = 10;
            break;

          case 7:
            _context.prev = 7;
            _context.t0 = _context['catch'](0);

            console.error(_context.t0);

          case 10:
            _context.prev = 10;

            console.log('finally');
            return _context.finish(10);

          case 13:
          case 'end':
            return _context.stop();
        }
      }
    }, _callee, this, [[0, 7, 10, 13]]);
  }));

  return function async2() {
    return _ref.apply(this, arguments);
  };
}();

function _asyncToGenerator(fn) { return function () { var gen = fn.apply(this, arguments); return new Promise(function (resolve, reject) { function step(key, arg) { try { var info = gen[key](arg); var value = info.value; } catch (error) { reject(error); return; } if (info.done) { resolve(value); } else { return Promise.resolve(value).then(function (value) { step("next", value); }, function (err) { step("throw", err); }); } } return step("next"); }); }; }

写法上直观了的，但在浏览器端，还是有些代价，生成的代码会多出不少。