es2017中的循环异步

    const arr = [1, 2, 3, 4, 5]

    const set = (item) => {
      return new Promise((res, rej) => {
        setTimeout(() => {
          res(item)
        }, 1000)
      })
    }

    const begin = async() => {
      for (let i of arr) {
        console.log(await set(i))
      }
    }