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