Error handling in an Express route - error-handling

Question

I have a Node module that wraps a RESTful API. This client follows the standard Node callback pattern:
module.exports = {
GetCustomer = function(id, callback) { ...}
}
I am calling this client from various Express routes like so:
app.get('/customer/:customerId', function(req,res) {
MyClient.GetCustomer(customerId, function(err,data) {
if(err === "ConnectionError") {
res.send(503);
}
if(err === "Unauthorized") {
res.send(401);
}
else {
res.json(200, data);
}
};
};
The issue is that I think it's not DRY to check for "ConnectionError" every time I call this client. I don't believe I can call res.next(err) because that will get sent back as a 500 error.
Is there a Node or Javascript pattern I am missing here? In C# or Java, I would throw the appropriate exception in MyClient.

Answer 1

You want to create error handling middleware. Here's an example from Express: https://github.com/visionmedia/express/blob/master/examples/error-pages/index.js
Here's what I use:
module.exports = function(app) {
app.use(function(req, res) {
// curl https://localhost:4000/notfound -vk
// curl https://localhost:4000/notfound -vkH "Accept: application/json"
res.status(404);
if (req.accepts('html')) {
res.render('error/404', { title:'404: Page not found', error: '404: Page not found', url: req.url });
return;
}
if (req.accepts('json')) {
res.send({ title: '404: Page not found', error: '404: Page not found', url: req.url });
}
});
app.use( function(err, req, res, next) {
// curl https://localhost:4000/error/403 -vk
// curl https://localhost:4000/error/403 -vkH "Accept: application/json"
var statusCode = err.status || 500;
var statusText = '';
var errorDetail = (process.env.NODE_ENV === 'production') ? 'Sorry about this error' : err.stack;
switch (statusCode) {
case 400:
statusText = 'Bad Request';
break;
case 401:
statusText = 'Unauthorized';
break;
case 403:
statusText = 'Forbidden';
break;
case 500:
statusText = 'Internal Server Error';
break;
}
res.status(statusCode);
if (process.env.NODE_ENV !== 'production' && process.env.NODE_ENV !== 'test') {
console.log(errorDetail);
}
if (req.accepts('html')) {
res.render('error/500', { title: statusCode + ': ' + statusText, error: errorDetail, url: req.url });
return;
}
if (req.accepts('json')) {
res.send({ title: statusCode + ': ' + statusText, error: errorDetail, url: req.url });
}
});
};