title: {ANSWERED} How do I look for an element with getElementById outside of the js file my code is in?
body: <p>I'm trying to use <code>document.getElementById</code> in a .js file external to my .html file. What word would I use instead of <code>document</code> to access it? 
  I get this error:</p>
  
  <pre><code>Uncaught TypeError: Cannot read property 'addEventListener' of null
  </code></pre>