微任务执行顺序

第一题：
链式Promise

    new Promise((resolve, reject) => {
      console.log(1)
      resolve()
    }).then(() => {
      console.log(2)
      new Promise((resolve, reject) => {
        console.log(3)
        resolve()
      }).then(() => {
        console.log(4)
      }).then(() => {
        console.log(5)
      })
    }).then(() => {
      console.log(6)
    })

    new Promise((resolve, reject) => {
      console.log(7)
      resolve()
    }).then(() => {
      console.log(8)
    })

  // 执行结果：1,7,2,3,8,4,6,5

第二题：
async / await

    setTimeout(() => {
      console.log(1)
    }, 100)

    setTimeout(() => {
      console.log(2)
    }, 0)

    Promise.resolve(console.log(3)).then(() => { console.log(4) })

    async function async1() {
      console.log(5)
      await async2()
      console.log(6)
    }

    async function async2() {
      console.log(7)
    }

    async1()

    // 结果：3, 5, 7, 4, 6, 2, 1

第三题：
宏任务嵌套微任务

    console.log(1)

    setTimeout(function() {
      console.log(2)
      new Promise(function(resolve) {
        console.log(3)
        resolve()
      }).then(function() {
        console.log(4)
      })
    });

    new Promise(function(resolve) {
      console.log(5)
      resolve()
    }).then(function() {
      console.log(6)
    })

    setTimeout(function() {
      console.log(7)
      new Promise(function(resolve) {
        console.log(8)
        resolve()
      }).then(function() {
        console.log(9)
      })
    });

    // 1 5 6 2 3 4 7 8 9 

第四题：

Promise.resolve().then(() => {
    console.log(0)
    return Promise.resolve(4)
}).then((res) => {
    console.log(res)
})

Promise.resolve().then(() => {
    console.log(1)
}).then(() => {
    console.log(2)
}).then(() => {
    console.log(3)
}).then(() => {
    console.log(5)
}).then(() => {
    console.log(6)
})

// 0, 1, 2, 3, 4, 5, 6