login2.ts

import { Component } from '@angular/core';
import { NavController, AlertController } from 'ionic-angular';
import { TabsPage } from '../tabs/tabs';

import { Auth } from '../../providers/auth';

@Component({
    selector: 'page-login',
    templateUrl: 'login.html'
})
export class LoginPage {
    credenciales = { usuario: '', contrasena: '' };

    constructor(public navCtrl: NavController, public alertCtrl: AlertController, public auth: Auth) { }

    iniciarSesion() {
        if (this.auth.iniciarSesion(this.credenciales)) {
            this.navCtrl.setRoot(TabsPage);
        } else {
            let alert = this.alertCtrl.create({
                title: 'Error',
                subTitle: 'Usuario y/o contraseña incorrecta.',
                buttons: ['Cerrar']
            });
            alert.present(prompt);
        }
    }
}