Aurelia-Validation: My approach at centralizing the validation and still being able to use custom-validation

Custom Validation

import moment from 'moment';

export function isValidDate(fmt) {
    let dateFormat = fmt || 'MM/DD/YYYY';

    this.passes((newValue) => {
        let mmt = moment(newValue, dateFormat);
        return mmt.isValid() && mmt.format(dateFormat) === newValue;
    });
    return this;
}

Model with Centralized Validation Logic and Custom Validation

import {inject, transient} from 'aurelia-framework';
import {ValidationGroup} from 'aurelia-validation';
import {isValidDate} from 'framework/validation/is-valid-date';

@transient()
@inject()
export class UserModel {

    constructor() {
        ValidationGroup.prototype.isValidDate = isValidDate;
    }

    firstName = "";
    lastName = "";
    email = "";
    password = "";
    confirmPassword = "";
    dob = "";
    gender = "";

    validator(item, validation) {
        return validation.on(item)
            .ensure('firstName')
                .isNotEmpty()
            .ensure('lastName')
                .isNotEmpty()
            .ensure('email')
                .isNotEmpty()
                .isEmail()
            .ensure('password')
                .isNotEmpty()
            .ensure('confirmPassword')
                .isNotEmpty()
            .ensure('dob')
                .isNotEmpty()
                .isValidDate()
                    .withMessage('not valid date')
            .ensure('password')
                .isIn(['Male','Femal'])
    }
}

View Model Calling Model

import {inject, computedFrom} from 'aurelia-framework';
import {Validation} from 'aurelia-validation';
import {UserFactory} from 'framework/factories/user-factory';
import moment from 'moment';


@inject(Validation, UserFactory)
export class AuthRegister {

    constructor(validation, userFactory) {
        this.userFactory = userFactory;
        this.user = this.userFactory.create();
        
        // Validation added here. Passes the item and validation object
        this.userValidation = this.user.validator(this.user, validation);
    }

    createUser() {
        // using promise array here because I will be validating more then one model
        let promises = [];
        
        promises.push(this.userValidation.validate());

        Promise.all(promises)
            .then(() => {
                console.log('both valid');
            })
            .catch(error => {
                console.log('something not valid');
                console.log(error);
            })
    }
}