for...of異步循環

• for…in && forEach && for 是常規的同步遍歷

• for…of常用於異步的遍歷

  function muti(num) {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve(num * num)
      }, 1000)
    })
  }

  let arr = [1, 2, 3];

  arr.forEach(async (i) => {
    const res = await muti(i)
    console.log(res)
  })

  (async function (){
    for (let i of arr) {
      console.log(await muti(i));
    } 
  }())