Sum Big Integers
<p>I'm currently stuck on a Codewars challenge that I can't get my head around:</p>

<p>Given a string representation of two integers, return the string representation of those integers, e.g. <code>sumStrings('1','2') // =&gt; '3'</code></p>

<p>I've used the following code so far, but it fails on large number test cases as the number is converted into a scientific notation:</p>

<pre><code>function sumStrings(a,b) {
  var res = +a + +b;
  return res.toString();
}
</code></pre>

<p>Any help would be much appreciated.</p>

<p><strong>Edit:</strong></p>

<p>Fiddle example: <a href="https://jsfiddle.net/ag1z4x7d/" rel="nofollow">https://jsfiddle.net/ag1z4x7d/</a></p>