Here is two method work for hardware and toolbar backbutton control

this method use for toolbar back button control

 import { Component,ViewChild  } from '@angular/core';
import { IonicPage, NavController, NavParams ,Platform,AlertController,Navbar } from 'ionic-angular';


  @ViewChild(Navbar) navBar: Navbar;
 this.navBar.backButtonClick = (e:UIEvent)=>{
     // todo something
     this.navCtrl.push(LoginPage);
    }

this method use for hardware back button control

import { Platform} from 'ionic-angular';

  this.platform.registerBackButtonAction(() => {
    //  this.showalert();
    },1);