title: node.js callback function does not return value
body: <p>I want to return err or result to postdata function but it's not working.<br>
   I am using mongoose database and collection name is 'Demo'. </p>
  
  <pre><code>  Demo.prototype.postdata = function(username, mobile, image) {
      var data = new conn.Demo({username, mobile, image});
      data.save(function(err, result) {
          if (err) {
              return err;
          } else {
              return result;
          }
      });
      return data.save();
  }
  </code></pre>