ES7值async+await分析

ES7：

const a = function(val) {
  return new Promise(function(resolve, reject) {
    if (val > 5) {
        resolve(val * val);
    } else {
        reject(val);
    }
  });
};
const b = async function () {
    const val = await a(6);
    console.log(val);
};

b();

转成ES5，可见底层是用promise来实现的：

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) { return step("next", value); }, function (err) { return step("throw", err); }); } } return step("next"); }); }; }

const a = function (val) {
  return new Promise(function (resolve, reject) {
    if (val > 5) {
      resolve(val * val);
    } else {
      reject(val);
    }
  });
};
const b = (() => {
  var ref = _asyncToGenerator(function* () {
    const val = yield a(6);
    console.log(val);
  });

  return function b() {
    return ref.apply(this, arguments);
  };
})();

b();