title: Angular - What is the meanings of module.id in component?
body: <p>In an Angular app, I have seen that <code>@Component</code> has property <code>moduleId</code>. What does it mean? </p>
  
  <p>And when <code>module.id</code> is not defined anywhere, the app still works. How can it still work?</p>
  
  <pre class="lang-javascript prettyprint-override"><code>@Component({
    moduleId: module.id,
    selector: 'ng-app',
    templateUrl: 'app.component.html',
    styleUrls: ['app.component.css'],
    directives: [AppComponent]
  });
  </code></pre>