javascript - How can handle the error in node.js? -

models/user.js

var user = module.exports = mongoose.model('user',userschema);  module.exports.getuserbyusername = function(username, callback){     var query = {username:username};     user.findone(query, callback); }  module.exports.createuser = function(newuser, callback){     user.getuserbyusername(newuser.username,function(err,username){         if(err) throw err;         // username exists         if(username){             // how can handle error?             console.log('username exists');         } else{             bcrypt.hash(newuser.password, 10, function(err,hash){                 if(err) throw err;                 newuser.password = hash;                 newuser.save(callback);             });         }     }); } 

router/users.js

router.post('/register',function(req,res,next){     var username = req.body.username;     var password = req.body.password;      var newuser = new user({         username:username,         password:password     });      req.checkbody('username','username field required').notempty();     req.checkbody('password','password field required').notempty();      var errors = req.validationerrors();      if(errors){         res.render('register',{             errors: errors,             username: username,             password: password         });     } else{         user.createuser(newuser,function(err,user){             if(err) throw err;             req.flash('success','you registered , may log in');             res.location('/');             res.redirect('/');         });     } }); 

above codes user registration code.

in models/user.js

i made function 'createuser' new user, , find whether exists. however, don't know how throw custom error, "user exists"..

what should do?

i tried :

// username exists if(username) {   callback(new error('username exists')); } 

but didn't work!

to throw error message "username exsits", should do?

thank reading. :)

p.s) models/user.js -> createuser function defined.

routes/users.js -> createuser function attached work.

you handle following node best practices , return error in callback

module.exports.createuser = function(newuser, callback){     user.getuserbyusername(newuser.username, function(err, username) {         if(err) throw err;          if(username){             callback('username exists', null); // callback(error, data) ...          ...... 

then catch it

router.post('/register',function(req,res,next){     ... data      if(errors){         res.render('register',{             errors: errors,             username: username,             password: password         });     } else {         user.createuser(newuser,function(err,user){             if(err) {                 res.render('register',{                     errors: err,         // username exists                     username: username,                     password: password                 });             } else {                 req.flash('success','you registered , may log in');                 res.location('/');                 res.redirect('/');             }         });     } });