title: AJAX: Check if a string is JSON?
body: <p>My JavaScript sometimes crashes on this line:</p>
  
  <pre><code>var json = eval('(' + this.responseText + ')');
  </code></pre>
  
  <p>Crashes are caused when the argument of <code>eval()</code> is not JSON. Is there any way to check if the string is JSON before making this call?</p>
  
  <p>I don't want to use a framework - is there any way to make this work using just <code>eval()</code>? (There's a good reason, I promise.)</p>