How To Display errors in a component view with Angular 2?

In Angular 1, the ng-messages modules are used to help us to display error messages and validation to our forms.

In Angular 2, the ngModel provides error objects for each of the built-in input validators. You can access these errors from a reference to the ngModel itself then build useful messaging around them to display to your users.

Example as,

import { Component, Input } from '@angular/core';
import { FormControl } from '@angular/forms';
import { ValidationService } from '../services/validation.service';

//The Component Class.
@Component({
  selector: 'kg-errorMessages',
  template: `<div *ngIf="errorMessage !== null">{{errorMessage}}</div>`
})
export class ErrorMessagesComponent {
  @Input() control: FormControl;
  @Input() name: string;

  constructor() { }

  get errorMessage() {
    for (let propertyName in this.control.errors) {
      if (this.control.errors.hasOwnProperty(propertyName) && this.control.touched) {
        return ValidationService.getValidatorErrorMessage(propertyName, this.control.errors[propertyName]);
      }
    }
    return null;
  }
}

//The Validator Services.

export class ValidationService {
    static getValidatorErrorMessage(validatorName: string, validatorValue?: any) {
        let config = {
            'required': 'Required!',
            'invalidNumberField': 'Only numbers allowed!',
            'invalidDateField': 'Not a valid date!',
            'invalidCreditCard': 'Is invalid credit card number!',
            'invalidEmailAddress': 'Invalid email address!',
            'invalidPassword': 'Password must be at least 6 characters long, and contain a number!',
            'invalidPasswords': 'Invalid passwords. Passwords must match!',
            'minlength': `Minimum length ${validatorValue.requiredLength}`
        };
        return config[validatorName];
    }

    static dateFieldValidator(control) {
        var e: boolean;

        if (IsValidDate(control.value)) {
            return null;
        } else {
            return { 'invalidDateField': true };
        }
    }

    static creditCardValidator(control) {
        // Visa, MasterCard, American Express, Diners Club, Discover, JCB
        if (control.value.match(/^(?:4[0-9]{12}(?:[0-9]{3})?|5[1-5][0-9]{14}|6(?:011|5[0-9][0-9])[0-9]{12}|3[47][0-9]{13}|3(?:0[0-5]|[68][0-9])[0-9]{11}|(?:2131|1800|35\d{3})\d{11})$/)) {
            return null;
        } else {
            return { 'invalidCreditCard': true };
        }
    }

    static emailValidator(control) {
        // RFC 2822 compliant regex
        if (control.value.match(/[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?/)) {
            return null;
        } else {
            return { 'invalidEmailAddress': true };
        }
    }

    static passwordValidator(control) {
        // {6,100}           - Assert password is between 6 and 100 characters
        // (?=.*[0-9])       - Assert a string has at least one number
        if (control.value.match(/^(?=.*[0-9])[a-zA-Z0-9!@#$%^&*]{6,100}$/)) {
            return null;
        } else {
            return { 'invalidPassword': true };
        }
    }

    static passwordCompareValidator(fg) {
        if (fg.value.password === fg.value.confirmPassword) {
            return null;
        } else {
            return { 'invalidPasswords': true };
        }
    }
}

References,

https://coryrylan.com/blog/angular-form-builder-and-validation-management

http://stackoverflow.com/questions/40494411/how-do-i-display-errors-in-an-component-view-with-angular-2

Stayed Informed - Angular 2 forms validation

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