title: How does += (plus equal) work?
body: <p>I'm a bit confused with the += sign. How does it work?</p>
  
  <ol>
  <li><p><code>1 += 2</code> // equals ?</p></li>
  <li><p>and this</p>
  
  <pre><code>var data = [1,2,3,4,5];
  var sum = 0;
  data.forEach(function(value) {
      sum += value; 
  });
  sum = ?
  </code></pre></li>
  </ol>