title: user defined function with callback in nodejs
body: <p>can anyone give me an example in which we are creating a particular function, which is also having a callback function ?</p>
  
  <pre><code>function login(username, password, function(err,result){
  });
  </code></pre>
  
  <p>where should I put the code of the login function and callback function?<br>
  p.s.: I am new to nodejs</p>