title: Join is not a function
body: <p>I'm playing around with <a href="http://pegjs.org/online" rel="noreferrer">PEG.js</a></p>
  
  <pre><code>start = keyword
  
  keyword = a:[a-z]? {return a.join("");}
  </code></pre>
  
  <p>Why am I getting here the error:</p>
  
  <blockquote>
    <p>a.join is not a function</p>
  </blockquote>
  
  <p>when I enter a valid string like <code>abc</code>?</p>