To close the app from particular page on single tap of hardware back button

I am posting the solution which worked for me, It may help for other users.

app.component.ts file should look like this:

import { Component } from '@angular/core';
import { Platform, ToastController } from '@ionic/angular';
@Component({
  selector: 'app-root',
  templateUrl: 'app.component.html'
})
export class AppComponent {
  counter = 0;

  constructor(
              public platform: Platform,
              public toastController: ToastController
  ) { 
    this.platform.ready().then(() => {
      document.addEventListener('backbutton', async () => {
          if (this.counter === 0) {
            this.counter++;
            this.alertToast();
            setTimeout(() => { this.counter = 0; }, 3000);
          } else {
            (navigator as any).app.exitApp();
          }
      });
    });
  }

  async alertToast() {
    const toast = await this.toastController.create({
      message: 'Press again to exit',
      duration: 300,
      position: 'middle',
    });
    toast.present();
  }


}