promise-then执行顺序问题

Posted 2022-05-27 沿着路走到底

以下代码，执行会输出什么

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

then 交替执行

如果有多个 fulfilled 状态的 promise 实例，同时执行 then 链式调用，

then 会交替调用

这是编译器的优化，防止一个 promise 持续占据事件

Promise.resolve().then(() => 
    console.log(1)
).then(() => 
    console.log(2)
).then(() => 
    console.log(3)
).then(() => 
    console.log(4)
)

Promise.resolve().then(() => 
    console.log(10)
).then(() => 
    console.log(20)
).then(() => 
    console.log(30)
).then(() => 
    console.log(40)
)

Promise.resol