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