title: JavaScript in operational throw a exception?
body: <p>code like this</p>
  
  <pre><code>&lt;script type="text/javascript"&gt;
    var name = [2,3,4];
    var a = (1 in name);
    console.log(a);
  &lt;/script&gt;
  </code></pre>
  
  <p>but throw error</p>
  
  <p><a href="https://i.stack.imgur.com/wSX2a.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wSX2a.png" alt="enter image description here"></a></p>
  
  <p>if code like this:</p>
  
  <pre><code>&lt;script type="text/javascript"&gt;
    var names = [2,3,4];
    var a = (1 in names);
    console.log(a);
  &lt;/script&gt;
  </code></pre>
  
  <p>no error! why?  test on firefox!</p>