javascript - Scope variables in typescript

I'm working with Angular 2 (Typescript) on Ionic 2 App. I have one class NewFavoriteSitePage with private property siteForm and I can use this property in class methods but when I am inside one google maps method this variable is undefined. What are the scope of variable or How do I define the variable to access from both sides?

declare var google;
......
export class NewFavoriteSitePage {
  .....
  private siteForm: FormGroup;

  loadMap(){

  //I can access to siteForm here!
  .....
      google.maps.event.addListener(marker, 'dragend', function(marker, siteForm){
        let newlatLng = marker.latLng; 
        console.log(this.siteForm); //Here this.siteForm is undefined
      });
  }