title: What's the difference between these two addEvent functions?
body: <p>This is the first:</p>
  
  <pre><code>function addEvent(el, type, fn){
      if(el.addEventListener){
          el.addEventListener(type, fn, false);
      }else{
          el[fn] = function(){
              fn.call(el, window.event);
          }
          el.attachEvent('on'+type, el[fn]);
      }
  }
  </code></pre>
  
  <p>and the second:</p>
  
  <pre><code>function addEvent(el, type, fn){
      if(el.addEventListener){
          el.addEventListener(type, fn, false);
      }else{  
          el['e' + fn] = function(){
              fn.call(el, window.event);
          }
          el.attachEvent('on'+type, el['e'+fn]);
      }
  }
  </code></pre>
  
  <p>the second one just add a prefix, what's it use for? </p>