xhr请求，koa处理，服务端能接受并顺利处理，但无法返回response，xhr.status=404，但将路由里的代码从向MongoDB获取数据 换成 简单的赋值返回，则一切正常，xhr.status=200.如下图，如果换成注释内的代码，则正常执行

route.post('/login',async function(ctx,next){

var userauth=function () {
     userModel.find({name: ctx.request.body.username}, function (err, user) {
        if (err) {
            throw err;
        }
        if (user.length == 0) {
            console.log(typeof(user));
            console.log(user.length == 0);
            console.log('user not register');
            return "user not register";
        }

        else {
            userModel.find({
                name: ctx.request.body.username,
                password: ctx.request.body.password
            }, function (err, user) {
                console.log(ctx.request.body.username);
                console.log(ctx.request.body.password);
                console.log(user);
                if (err) {
                    throw err;
                }
                console.log("user:" + user)
                if (user == null || user.length == 0) {
                    console.log(user);
                    console.log('password wrong');
                    ctx.response.body = "login fail";
                    return "login fail";
                }
                if (user != null && user.length > 0) {
                    console.log('login sccessful');
                    ctx.response.body = "login successful";
                    return "login successful";
                }
            })
        }

    })
}
var response=await userath();
return response;

/*var initial={ username:"suki",password:'123456'};
console.log(ctx.request.body);
console.log(ctx.request.body.username);
console.log(ctx.request.body.password);
if(ctx.request.body.username==initial.username){
    console.log("登录成功");
    ctx.response.body="login successful";
    //return response;
}
else {
    console.log("登录失败");
    ctx.response.body="login fail";
}
console.log("post login end");*/

})

请问是为什么？以及解决方法。谢谢你们啦~~