then
then函数可以return另一个promise:
const p1 = new Promise((resolve, reject) =>{
resolve('p1')
})
const p2 = new Promise((resolve, reject) =>{
setTimeout(() =>{
resolve('p2')
},3000)
})
p1.then(res => {
console.log(res)
return p2
}).then(res =>{
// p2 resolve后才执行
console.log(res)
})
//p1