title: How is Meteor's withTracker function executed differently than the former reactive container function createContainer?
body: <p>The following is an example of <code>withTracker()</code> function call which includes usage of sequential parentheses in Javascript.</p>
  
  <pre><code>export default withTracker(props =&gt; {
      const handle = Meteor.subscribe('todoList', props.id);
      return {
          currentUser: Meteor.user(),
          listLoading: !handle.ready(),
          tasks: Tasks.find({ listId: props.id }).fetch(),
      };
  })(Foo);
  </code></pre>
  
  <p>The former reactive container for React components in Meteor was the <code>createContainer()</code> function and it was called like the following for the same purpose of the above one.</p>
  
  <pre><code>export default FooContainer = createContainer(props =&gt; {
    const handle = Meteor.subscribe('todoList', props.id);
    return {
      currentUser: Meteor.user(),
      listLoading: ! handle.ready(),
      tasks: Tasks.find({ listId: props.id }).fetch(),
    };
  }, Foo);
  </code></pre>
  
  <p>What is the difference in the execution of these two functions? </p>