问User.findOne()不是函数

EN

passport.use('local-signup', new LocalStrategy({
    // by default, local strategy uses username and password, we will override with email
    usernameField : 'email',
    passwordField : 'password',
    passReqToCallback : true // allows us to pass back the entire request to the callback
},
(req, email, password, done) => {
    // asynchronous
    // User.findOne wont fire unless data is sent back
    process.nextTick(() => {

    // find a user whose email is the same as the forms email
    // we are checking to see if the user trying to login already exists
    User.findOne({ 'email' :  email },function(err, user){
        // if there are any errors, return the error

        if (err)
            return done(err);

        // check to see if theres already a user with that email
        if (user) {
            return done(null, false, {'errorMessages': 'That email is already taken.'});
        } else {

            // if there is no user with that email
            // create the user
            let newUser            = new User();

            // set the user's local credentials
            newUser.name       = req.body.fullname;
            //newUser.email          = email;
            newUser.password       = newUser.generateHash(password);


            // save the user
            newUser.save((err)=>{
                if (err)
                    return done(err);
                return done(null, newUser);
            });
        }
    });    
    });
}));