promise then里面使用return的疑问？

//1
exports.login = function* (){
    let data = {config};
    let req = this.request.body;
    let loginname = req.loginname;
    let password = md5(req.password);

    yield Promise.resolve()
    .then(() => User.findOne({loginname: loginname}))
    .then((user) => {
        if(user && (password === user.password)){
            this.session.loginname = loginname;
            this.redirect('/');
        }else{
            data.errMsg = 'please check name & password';
            return co(this.render('login', data));
        }
    });
};

//2
exports.userInfo = function* (){
    let loginname = this.params.loginname;
    let data = {
            config,
            user: this.session.user
        };

    yield Promise.resolve()
    .then(() => User.findOne({loginname: loginname}))
    .then((user) => {data.userintro = user;})
    .then(() => co(this.render('user', data)))
};

这是使用koa编写的两端代码，为何同样是then里面写的函数，一个需要return，而另一个不需要return,也能渲染页面？

---

当只有一条语句例如 () => co(this.render('user', data)) 时，等价于

function (){ return co(this.render('login', data));}

当 ()=>{} {}内有多条语句，须加return

---

() => someobj

等价于

function () {return someobj}