title: Add property to all objects in array
body: <p>I have the following array of objects:</p>
  
  <pre><code>var array = [ {'a': '12', 'b':'10'}, {'a': '20', 'b':'22'} ];
  </code></pre>
  
  <p>How can I add a new property <code>c = b - a</code> to all objects of the array?</p>