What does => do and what is it called in the context below? : <p>What does (a=>) do in the context below?</p>

<pre><code>function findOutlier(int){
  var even = int.filter(a=&gt;a%2==0);
  var odd = int.filter(a=&gt;a%2!==0);
  return even.length==1? even[0] : odd[0];
}
</code></pre>
