return from bcrypt.compare

Question

I am currently making an authentication server, but no matter what I do, I can not get this function to return something. I would like this function to return ok if the hashed password compares to the userpassword and return invalid password or User does not exist if it does not.

function authenticateUser(username, password){
    user = User.findOne({username});
    if(!user) {
        return 'User does not exist.';
    }
    bcrypt.compare(password, user.password, (err,success)=>{
        if(err){
            return 'invalid password';
        }
        else {
            return 'ok';
        }
    });
}