UserSchema.pre('save', function(next) {
    let user = this;
    if(!user.isModified('password')) return next(); //password has not modified
    bcrypt.genSalt(10, function (err, salt) {
        bcrypt.hash(user.password, salt, function (err, hash) {
            if(err) return next(err);
            user.password = hash;
            next();
        });
    });
});


UserSchema.methods.comparPassword = function(candidatePassword, cb) {

    bcrypt.compare(candidatePassword, this.password, function(err, isMatch) {
        if(err) return cb(err);
        cb(null, isMatch);