title: Javascript run a function inside a loop every x iterations
body: <p>I have a variable of unknown value, it will be an integer. For this sake, lets say <code>var a = 3;</code></p>
  
  <p>I have a function that is called continuously:</p>
  
  <pre><code>var a = 3;
  function anim() {
          left = parseInt(galleryInner.css('left'), 10);   
          if(Math.abs(left) &gt;= (galleryItem.length * galleryItem.width())){
              galleryInner.css('left', 0);
          }
          galleryInner.animate({left: '-=20' }, 200, anim);
  
          that.appendEnd();
  }
  </code></pre>
  
  <p>I'd like run this.appendEnd() only every 3 times, because <code>a === 3</code>.</p>
  
  <p>How can I do this?</p>