async & await

 async & await简单来说就是 iterator(const a = function* (){} & yield & a.next()) 的升级版，更简洁。

用简单写法将异步函数同步化，好用得很啊

   const asyncFn = function(time) {
      return new Promise((resolve, reject) => {
        setTimeout(function() {
          resolve(time)
          console.log(time);
        }, time)
      })

    }
    const testTimeOut = async function() {
      const time1 = await asyncFn(1000);
      const time2 = await asyncFn(500);
      console.log(time1, time2);//1000,500
    }

    testTimeOut();