javascript - ngOnInit から関数に値を渡す

Question

クリップ機能を呼び出すボタンがあります。ngOnInitからその関数に値を渡す必要がありますが、できません。(たとえば) this.firstXに対して未定義のエラーが発生します。ngOnInit から関数に値を渡すにはどうすればよいですか?

export class AppAppComponent implements OnInit {

  public firstX: number;
  public firstY: number;
  public lastX: number;
  public lastY: number;

  constructor(mdIconRegistry: MdIconRegistry) {
    mdIconRegistry
      .registerFontClassAlias('fontawesome', 'fa');
  }

  ngOnInit() {

    let canvas = new fabric.Canvas('cLeft');
    let src = "../../images/kupurSecond.jpg";

    canvas.on('mouse:down', function (event) {
      var position = canvas.getPointer(event.e);
      this.firstX = position.x;
      this.firstY = position.y;
    });

    canvas.on('mouse:up', function (event) {
      var position = canvas.getPointer(event.e);
      this.lastX = position.x;
      this.lastY = position.y;
      // ReLoading Image
      loadImage(src, canvas, this.firstX, this.firstY, this.lastX, this.lastY);
    });

    loadImage(src, canvas);

  }; 

  public clip() {
    console.log(this.firstX);
  }

}