index.js

var userSchema = new Schema({
    email: String
  , passwordHash: String
});

userSchema.pre('validate', function(next) {
  if (this.password === this.passwordConfirm) {
    this.set('passwordHash', hash(this.password);

    // How do I prevent this.password and this.passwordConfirm
    // to be save into DB?
  }

  next()
});

var User = db.model('User', userSchema);

var user = new User()
user.init({
    email: 'hello@example.com'
  , password: 'password'
  , confirmPassword: 'password'
});

user.save(function(err) {
  console.log(!!err ? 'error' : 'saved');
});

function hash(str) { return str+'hashed' }