深入解析：为什么async/await被称为Generator函数的语法糖

function sleep(time) {
 return new Promise((resolve, reject) => {
 setTimeout(() => {
  resolve(1);
 }, time);
 });
}

async function test () {
 for(let i = 0; i < 10; i++) {
 let result = await sleep(1000);
 console.log(result);
 }
}

let test = function () {
 // ret 为一个Promise对象，因为ES6语法规定 async 函数的返回值必须是一个 promise 对象
 let ret = _asyncToGenerator(function* () {
 for (let i = 0; i < 10; i++) {
  let result = yield sleep(1000);
  console.log(result);
 }
 });
 return ret;
}();

// generator 自执行器
function _asyncToGenerator(genFn) {
 return new Promise((resolve, reject) => {
 let gen = genFn();
 function step(key, arg) {
  let info = {};
  try {
  info = gen[key](arg);
  } catch (error) {
  reject(error);
  return;
  }
  if (info.done) {
  resolve(info.value);
  } else {
  return Promise.resolve(info.value).then((v) => {
   return step('next', v);
  }, (error) => {
   return step('throw', error);
  });
  }
 }
 step('next');
 });
}