title: Sum of Digits / Digital Root codewars kata
body: <p>Please, help me. I don't understand. Why second solution returns undefined?</p>
  <pre><code>function digital_root(n) {
    return n &lt; 10 ? n : digital_root(n.toString().split('').map(Number).reduce((a, b) =&gt; a + b));
  }
  console.log(digital_root(12345678));
  // expected output: 9
  
  function digital_root(n) {
    if (n &lt; 10) {
      return n;
    } else {
      digital_root(n.toString().split('').map(Number).reduce((a, b) =&gt; a + b));
    }
  }
  console.log(digital_root(12345678));
  // output: undefined
  </code></pre>