title: Map over object preserving keys
body: <p>The <code>map</code> function in underscore.js, if called with a javascript object, returns an array of values mapped from the object's values.</p>
  
  <pre><code>_.map({one: 1, two: 2, three: 3}, function(num, key){ return num * 3; });
  =&gt; [3, 6, 9]
  </code></pre>
  
  <p>is there a way to make it preserve the keys? ie, I want a function that returns</p>
  
  <pre><code>{one: 3, two: 6, three: 9}
  </code></pre>