title: What's the difference between mixin() and extend() in Javascript libraries
body: <p>I'm looking through various libraries, and seeing extend() pop up a lot, but I'm also seeing mixin() show up. YUI has both mixins and extensions.</p>
  
  <p>What's the difference between these two concepts? When would I decide between a mixin and extending an object?</p>
  
  <p>Thanks,
  Matt</p>