title: callback?: (any) => void what this code mean?
body: <p>i'm a newbie in javascript and angular, so i'm learning these.
  could anybody interpret below code snippet for me?</p>
  <pre><code>public loadDataForPage(page: number, pageSize: number, callback?: (any) =&gt; void) {
     this._http.get(this._buildDataUrl(page, pageSize))
     .subscribe((data: any) =&gt; {
        const startIndex = (page-1) * pageSize;
        this._updateData(data, startIndex);
        this._data.next(data);
        callback({data});
     });
  }
  </code></pre>
  <p>i don't know this part
  <em><strong>callback?: (any) =&gt; void           and callback({data});</strong></em></p>
  <p>thanks.</p>