readLink(url: string) {
this.isBusy = true;
this._http.get(`${this._helper.baseURL}/api/post/readlink?url=${encodeURIComponent(url)}`)
  .subscribe(
  data => {
    this.processResponse(data, url);
  },
  (err: HttpErrorResponse) => {
  }
  );
 }

processResponse(result, url) {
if (result.images != null && result.images.length > 0) {
  result.images.forEach(el => {
    this.getMeta(el, function (res) {
      this.images.push(res);
      this.currentImage = this.images[0];
    });
  });
}
alert( this.images.length); // this always give zero and this was executed before my call bakc
}

getMeta(url, callback) {
const img = new Image();
img.src = url;
img.onload = function () {
  if (img.width > 260 && img.height > 150) {
    callback(url);
  }
};