mapItem(item: any)
{
  //do something
}

mapItems(items: any[])
{
  for (const item of items)
    this.mapItem(item);
}

callServer()
{
  return this.httpServer.get(url).map(this.mapItems);
}

import { map } from "rxjs/operators";

callServer()
{
  return this.httpServer.get(url).pipe(map(this.mapItems));
}