1-Currently.js

User.get({username: req.body.username}, function(err, user) {
    if(err) {return console.log(err);}

    if(user) {
      res.status(404).json({error: ["Username already exist."]});
    } else {
        var user = new User({username: req.body.username, provider: 'local', email: req.body.email, password: bcrypt.hashSync(req.body.password, 10)});

        user.put(function(err, success){
          if(err) {return console.log(err)}

          var token = jwt.encode(user, 'lkmaspokjsafpaoske1a8asda0s9a');
          res.json({token: token, user: user});
        });
      }
  });

2-Cleaner.js

User.get({ username: req.body.username }, function(err, user) {
    if (err) {
      return res.status(400).json({ error: ["Error when fetching user."] });
    }

    if (user) {
      return res.status(404).json({ error: ["Username already exist."] });
    }
    
    var user = new User({ username: req.body.username, provider: 'local', email: req.body.email, password: bcrypt.hashSync(req.body.password, 10) });
    user.put(function(err, success){
      if (err) {
        return res.status(500).json({ error: ["Error when creating user."] });
      }

      var token = jwt.encode(user, 'lkmaspokjsafpaoske1a8asda0s9a');
      return res.json({ token: token, user: user });
    });
  });

3-Cleanest-ES6-Async.js

User
  .getAsync({ username: req.body.username })
  .then(user => {
    if (user) {
      return res.status(404).json({ error: ["Username already exist."] });
    }
    
    const user = new User({ 
      username: req.body.username, 
      provider: 'local', 
      email: req.body.email, 
      password: bcrypt.hashSync(req.body.password, 10) 
    });
  
    return user
      .putAsync()
      .then(success => res.json({ token: jwt.encode(user, 'lkmaspokjsafpaoske1a8asda0s9a'), user: user }));
  })
  .catch(err => res.status(500).json({ error: ["There was an error processing your request."] }));