title: setInterval behaviour in TypeScript
body: <p>I just started to play around with TypeScript.  I´ve created a sample project with Visual Studio 2012 Express for Web and this sample has a line of code which has a behaviour that I cannot explain myself.</p>
  
  <p>First the TypeScript code:</p>
  
  <pre><code>start() {
      this.timerToken = setInterval(() =&gt; this.span.innerHTML = new Date().toUTCString(), 500);
  }
  </code></pre>
  
  <p>So this line sets the value of timerToken every 500ms and updates a HTML element with the current Date/Time.</p>
  
  <p>In JavaScript that would be equivalent to this:</p>
  
  <pre><code>Greeter.prototype.start = function () {
          this.timerToken = setInterval(this.span.innerHTML = new Date().toUTCString(), 500);
      };
  </code></pre>
  
  <p>So I wondered about the lambda expression in the TypeScript code line and removed it but then the Date/Time string won't be updated anymore.</p>
  
  <p>So easy question ... why?</p>