title: Javascript ReferenceError: Observable is not defined when added Rx.js in src
body: <p>I am trying to get observables to work on my html but I keep getting this error:</p>
  
  <p>ReferenceError: Observable is not defined</p>
  
  <p>I am using <code>Firefox version: 62.0</code> </p>
  
  <p>Here is the code:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;!doctype html&gt;
  &lt;html lang="en"&gt;
  &lt;head&gt;
      &lt;meta charset="utf-8"&gt;
      &lt;script type="text/javascript"  src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.4.0/Rx.js"&gt;&lt;/script&gt;
      &lt;script&gt;
       const numberObservable = new Observable((observer) =&gt; {
           observer.next(5);
           observer.next(10);
       });
      
       numberObservable.subscribe(value =&gt; console.log(value));
      &lt;/script&gt;
      &lt;title&gt;&lt;/title&gt; 
  &lt;/head&gt;
  &lt;body&gt;
  &lt;/body&gt;
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>How can I get this to work?</p>