Express.js - display a custom 404 page when a record is not found in MongoDB

Question

Express.js - display a custom 404 page when a record is not found in MongoDB

I am using node-mongodb-native driver. I tried

collection.findOne({email: 'a@mail.com'}, function(err, result) {
  if (!result) throw new Error('Record not found!');
});

But the error is caught by mongodb driver and the express server is terminated.

What's the correct way for this case?

=== Edit===

I have the code below in app.js

app.configure('development', function() {
    app.use(express.errorHandler({dumpExceptions: true, showStack: true}));
});

app.configure('production', function() {
    app.use(express.errorHandler());
});

Related code in node_modules/mongodb/lib/mongodb/connection/server.js

connectionPool.on("message", function(message) {
    try {
        ......
    } catch (err) {
      // Throw error in next tick
      process.nextTick(function() {
        throw err; // <-- here throws an uncaught error
      })
    }      
});

node.js
mongodb
express

Answer 1

The correct use is not to throw an error, but to pass it to next function. First you define the error handler:

app.error(function (err, req, res, next) {
    res.render('error_page.jade');
})

(What's this talk about error being depracated? I don't know anything about that. But even if then you can just use use. The mechanism is still the same.).

Now in your route you pass the error to the handler like this:

function handler(req, res, next) {
    collection.findOne({email: 'a@mail.com'}, function(err, result) {
        if (!result) {
            var myerr = new Error('Record not found!');
            return next(myerr); // <---- pass it, not throw it
        }
        res.render('results.jade', { results: result });
    });
};

Make sure that no other code (related to the response) is fired after next(myerr); (that's why I used return there).

Side note: Errors thrown in asynchronous operations are not handled well by Express (well, actually they somewhat are, but that's not what you need). This may crash your app. The only way to capture them is by using

process.on('uncaughtException', function(err) {
    // handle it here, log or something
});

but this is a global exception handler, i.e. you cannot use it to send the response to the user.

Answer 2

I'm guessing that the error is not caught. Are you using an Express error handler? Something like:

app.error(function (err, req, res, next) {
 res.render('error-page', {
  status: 404
 });

More on error handling in Express: http://expressjs.com/guide.html#error-handling

Answer 3

In terms of checking for errors off mongodb, use '!error' for success as opposed to '!result' for errors.

collection.findOne({email: 'a@mail.com'}, function(err, result) {
    if (!error) {
        // do good stuff;
    } else {
        throw new Error('Record not found!');
    }
});

As for the custom 404, I've yet to do that in node and express, but I would imagine it would involve "app.router".