User.pre('save', wrap(async () => {
    var user = this;
    if (!user.isModified('password')) return;
    var salt = await bcrypt.genSalt(10);
    var hash = await bcrypts.hash(user.password, salt, null);
    user.password = hash;
}));

const wrap = (fn) => {
    return (next) => {
        fn().then(() => {
            next();
        }, next);
    };
};