title: How to tell if a JavaScript function is defined
body: <p>How do you tell if a function in JavaScript is defined?</p>
  
  <p>I want to do something like this</p>
  
  <pre><code>function something_cool(text, callback) {
      alert(text);
      if( callback != null ) callback();
  }
  </code></pre>
  
  <p>But it gets me a</p>
  
  <blockquote>
    <p>callback is not a function</p>
  </blockquote>
  
  <p>error when callback is not defined.</p>