title: Count the number of Friday's that fall on the 13th within a given year
body: <p>I would like to create a function that goes through the months of a given year, calculates how many Fridays fall on the 13th, and returns that number. So far this is what I have:</p>
  
  <pre><code>function numberOfFridaythe13thsIn(jahr){
      var d = new Date();
      d.setFullYear(jahr, 0, 13);
      var counter = 0;
      var months = 0;
      while(months &lt;= 11) {
          months++;
          if(d.getDay() == 5 &amp;&amp; d.getDate() == 13) {
            counter++;
         }
      }
      return counter;                            
  }
  </code></pre>
  
  <p>I am imagining this code starts on January 13th of a given year, has a counter where the sum of days will go, and will cycle through the months. I know my code is off, but can I get some guidance?</p>