title: how to calculate button click count
body: <pre><code>let clickStream = Observable.fromEvent(button,'click');
  clickStream.scan( count =&gt; count + 1,0)
          .subscribe( count =&gt; console.log(`you click count is${count}!`));
  </code></pre>
  
  <p>this is angular2 project i'm use rxjs version is 5.0 , but count is object so it can't to calculate sum , how i to calculate count when button click?  this is log error ↓</p>
  
  <p>Operator '+' cannot be applied to types '{}' and 'number'.</p>