UserSchema.pre('save', function(next) {
    const user = this;
    if (this.isModified('password') || this.isNew) {
        return bcrypt.hash(user.password, 10)
            .then((hash) => {
                user.password = hash;
                return next();
            })
            .catch(err => next(err));
    }
    return next();
});