propagating error from a callback in nodes - node.js

Question

I'm a newbie as far as node is concerned, so what i am asking here might be plain out stupid, so bear with me.
Below is the code for a node module that connects to a given mongodb database, my problem is on line19 where i am trying to throw the error in case the connection to the db server cannot be made or the db server is down, but node complains , please advise.
Code:-
var dbinit_func = function(db_name){
try{
// require mongoose , if it's not there
// throw an exception and but out
var mongoose = require('mongoose');
}
catch(err){
throw "Error Mongoose Not Found"
}
try{
// connect to the db
mongoose.connect("mongodb://localhost/" + db_name);
// get a reference to the connection object
var db_connection = mongoose.connection;
// subscribe to events on the connection object
db_connection.on('error', function(err){
// holy cow "A Connection Error", shout it out loud
// and but out
throw new Error(err.message);--> This is where the problem Occurs
});
// bind to the connection open event , we just need to
// do it once , so we use the once method on the
// connection object
db_connection.once('open', function(){})
}
catch(err){
// we got an error most probably a connection error
// so we but out from here
throw "Connection Error";
}
}
module.exports = dbinit_func;
Message spitted by Node:-
/Users/tristan625/projects/node_projs/school_web/models/db.js:19
throw new Error(err.message);
^

Answer 1

Your code is written in a synchronous style, as seen by the try/catch blocks. What you must understand, is that the mongoose.connect function is asynchronous. When you call throw new Error(), that call will occur inside of a callback, which is outside the scope of your try/catch blocks, hence you can't handle the error using them.
Your dbinit_func must be written to allow for a callback
Here's a pretty standard way of doing what you're doing
function dbinit_func(db_name, callback) {
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/ + db_name', { auto_reconnect: true });
var db = mongoose.connection;
db.once('error', function(err){
//trigger callback with an error
callback(err);
});
db.once('open', function() {
//trigger callback with success
callback(null, 'DB Connection is open');
});
}
module.exports = dbinit_func;
// to call this function, you'd want to do something like this
var dbinit = require('/path/to/your/module');
dbinit('mydatabasename', function(err, res){
// at this point, there will either be an error
if (err) console.log(err.message);
// else it will be success
console.log(res);
});