title: How to notify front-end from backend?
body: <p>I am using multer for file uploading. On the server I have this:</p>
  
  <pre><code>stream.on('finish', () =&gt; {                                                                              
    console.log('finished')                                                                                
    next()
  })
  </code></pre>
  
  <p>What would be the best way (with what command) to notify the frontend that 'finished' has happened? I want to know this in order to have a loading bar.</p>