Node.js callback error pattern

Question

The errors are passed through every callback, but in fact, it might be better to throw an error at higher levels. For example, look at the mongodb.js database function. It passes through an error in the callback. However, if there was an error at that level, it might be better to deal with it at that point and just fall back on procces.on('uncaughtException')

It is a lot of extra code each time, dealing with errors in callbacks. I am wondering if there is a better way around it.

controllers/users.js

var usersModel = require('../models/users.js');
users.create = function(req, res){
 usersModel.create(req.data.username, req.data.password, function(error, user){
 if(error){
 res.statusCode = 500;
 res.end('server-error');
 return;
 }
 res.statusCode = 201;
 res.end(user._id);
 });
};

models/users.js

var db = require('../mongodb.js');
module.exports.create = function(username, password, callback){
 db.collection('main', 'users', function(error, collection){
 if(error){
 callback(error);
 return;
 }
 collection.insert({
 username: username,
 password: password,
 createdOn: new Date()
 }, callback);
 });
};

mongodb.js

var mongo = require('mongodb')
 , mongodb = module.exports
 , connections = {}
 , config = {
 main: {
 name: 'application-main',
 server: new mongo.Server('127.0.0.1', 27017, { auto_reconnect: true })
 }
 };
mongodb.database = function(key, callback){
 if(connections[key]){
 callback(null, connections[key]);
 return;
 }
 var db = mongo.Db(config[key].name, config[key].server);
 db.open(function(error, connection){
 if(error){
 callback(error);
 return;
 }
 connection.on('close', function(){
 delete(connections[key]);
 });
 connections[key] = connection;
 callback(null, connection);
 }
};
mongodb.collection = function(dbKey, collName, callback){
 mongodb.database(dbKey, function(error, connection){
 if(error){
 callback(error);
 return;
 }
 connection.collection(collName, function(error, collection){
 if(error){
 callback(error);
 return;
 }
 callback(null, collection);
 });
 });
};

• jackdbernier: Have you tried github.com/caolan/async#waterfall ? I would use it and handle errors in the callback.

Answer 1

First, there is a principle you should be reminded: Never deal with the error you don't know, just pop it up.

Which means, if you can't deal with or don't know about the error passed in the callback, you just pass it to next callback, or throw it(A pattern of CHAIN OF RESPONSIBILITY)!

For the errors you know how to deal with it, you can define a module for error handling and deal with it according to the type of error and log the error for debugging, like the following way.

error-handler.js

var log = require('./log'); // Log module(local or third-party);
function handler(error){
 if(error instanceof error1){
 // log it
 // handler for error1
 }else if(error instanceof error2){
 // log it 
 // handler for error2
 }else if ....
};
module.exports = handler;

logic.js

var errorHandler = require('./error-handler');
db.open(function(err, connection){
 if(err){ return errorHandler(err); }
});

For good modules like mongoose will throw their customized Errors.