title: TypeError [ERR_INVALID_CALLBACK]: Callback must be a function. Received undefined in nodejs
body: <p>Hi Guys am new in node js , i am getting an error when am trying to append my file in node js don't know what's going on wrong please try to fix my error and also tell me what should i do?</p>
  <pre><code>console.log(&quot;Starting app.js&quot;);
  
  const fs = require('fs');
  const os = require('os');
  const notes = require('../notes-node/notes.js');
  
  var user = os.userInfo();
  
  fs.appendFile('message.txt', `Hello ${user.username}! You are ${notes.age}`)
  
  console.log(user);
  </code></pre>