promise不会被return触发， 一个promise对象中不会被Promise.reject触发
1.
let a = new Promise((resolve,reject)=>{
return 23
})
a; // promise <pending>
2.
let a = new Promise((resolve,reject)=>{
Promise.reject(234)
})
a; // promise <pending>
3.
let a = new Promise((resolve,reject)=>{
return Promise.reject(12)
}
a; //promise <pending>
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/reject
