title: Instantiate a Class dynamically via variable
body: <p>How can I instantiate a class by throwing in a variable name? 
  Consider this method inside a class: </p>
  
  <pre><code>animate: function(el, build) { 
          console.log(build.effect); 
          var animationClass = new build.effect(el,build); 
  }, 
  </code></pre>
  
  <p>Build is an object containing lots of stuff, but most importantly an 
  "effect".  This effect is the name of an independent animation class-- 
  one is called "MarioKartMenu". </p>
  
  <p>console.log(build.effect) prints out "MarioKartMenu". 
  But of course I get: TypeError: Result of expression 'build.effect' [MarioKartMenu] is not a constructor.</p>
  
  <p>If I trash the dynamism and just make the code as such:</p>
  
  <pre><code>animate: function(el, build) {
          var animationClass = new MarioKartMenu(el,build);
      }, 
  </code></pre>
  
  <p>It works just fine.  Is it possible to make it dynamic like I'm attempting to do?</p>