title: How to map only every second value in array
body: <p>I have some array of numbers:</p>
  
  <pre><code>var arr = [1, 7, 1, 4];
  </code></pre>
  
  <p>I want to increase only every first value, such that the expected output would be: <code>[2, 7, 2, 4]</code></p>
  
  <p>I tried some combination of <code>map</code> and <code>filter</code> but I don't understand how it can work together...</p>
  
  <pre><code>var mapuj = arr.map(x =&gt; x *2);
  </code></pre>