title: Merge objects with the same id in an array by adding their numeric properties
body: <p>I already searched for a tutorial in Google but couldn't find what I needed.
  I have an array like this:</p>
  
  <pre><code>[
   {Username:'user1', Balance:'123'}, 
   {Username:'user2', Balance:'213'}, 
   {Username:'user1', Balance:'424'}
  ]
  </code></pre>
  
  <p>How to merge <code>Balance</code> when the <code>Username</code> is the same, for example merge <strong>Balance</strong> <code>Username:'user1'</code></p>