title: _.isUndefined implementation
body: <p>Why is underscore.js's isUndefined defined this way? </p>
  
  <p><code>_.isUndefined = function(obj) {
      return obj === void 0;
    };</code></p>
  
  <p>Why can't this work?</p>
  
  <p><code>typeof obj === 'undefined'</code></p>