Typescript Error: Cannot find name 'cordova'

import {Component} from '@angular/core';
import {NavController, Platform, AlertController} from 'ionic-angular';
import {Transfer, TransferObject} from '@ionic-native/transfer';
import {File} from '@ionic-native/file';

declare let cordova: any;

@Component({
  selector: 'page-about',
  templateUrl: 'about.html',
  providers: [Transfer, TransferObject, File]
})
export class AboutPage {

  storageDirectory: string = '';

  constructor(public navCtrl: NavController, public platform: Platform, private transfer: Transfer, private file: File, public alertCtrl: AlertController) {
    this.platform.ready().then(() => {
      // make sure this is on a device, not an emulation (e.g. chrome tools device mode)
      if(!this.platform.is('cordova')) {
        return false;
      }

      if (this.platform.is('ios')) {
        this.storageDirectory = cordova.file.documentsDirectory;
      }
      else if(this.platform.is('android')) {
        this.storageDirectory = cordova.file.externalDataDirectory;
		console.log(this.storageDirectory);
      }
      else {
        // exit otherwise, but you could add further types here e.g. Windows
        return false;
      }
    });
  }
  
   downloadImage() {

    this.platform.ready().then(() => {

      const fileTransfer: TransferObject = this.transfer.create();

      const imageLocation = 'http://html5demos.com/assets/dizzy.mp4';

      fileTransfer.download(imageLocation, this.storageDirectory + 'dizzy.mp4').then((entry) => {
       
	   const alertSuccess = this.alertCtrl.create({
          title: `Download Succeeded!`,
          subTitle: `successfully downloaded to: ${entry.toURL()}`,
          buttons: ['Ok']
        });

        alertSuccess.present();

      }, (error) => {

        const alertFailure = this.alertCtrl.create({
          title: `Download Failed!`,
          subTitle: `was not downloaded. Error code: ${error}`,
          buttons: ['Ok']
        });

        alertFailure.present();

      });

    });

  }


}