title: jQuery .map() returning without undefined
body: <p>I have an object array and I want to create a new array with the <code>id</code>s only. Some records have <code>id</code> and others don't.</p>
  
  <p>So I have something like:</p>
  
  <pre><code>var myMap = arr.map(function(e) {
      return e.id;
  });
  
  console.log(myMap); // [undefined, 2, 3, 4]
  </code></pre>
  
  <p>I'd like it to return just <code>[2, 3, 4]</code>, if possible.</p>
  
  <p>This JSFiddle should explain a little better: <a href="http://jsfiddle.net/dmathisen/Lnmj0w8k/" rel="nofollow">http://jsfiddle.net/dmathisen/Lnmj0w8k/</a></p>