What is the advantage of catching errors and responding from within a middleware function like in example A.
Example A
function(req, res, next)
{
if (err) {
err.message = 'Not Found';
res.status(404).json(err);
return;
}
}
Versus passing the error to the error handling middleware like in example B.
Example B
function(req, res, next)
{
if (err) {
var err = new Error('Not Found');
err.status = 404;
next(err);
}
}
error handling middleware:
app.use(function(err, req, res, next) {
res.status(err.status || 500);
res.json({message: "error"});
});
Imagine you are piping the request through several functions to process it, and the error in one of the first functions is superficial. You can still process the request, but you need to send a warning or handle the error afterwards. Perhaps you want to gather all of the errors in the pipeline report them back. Example B allows this.
But imagine the error is critical, and you know you won't be able to proceed. In that case you, can't call next, because the next function won't know how to deal with a fundamentally broken request. Here is where example A comes in handy.
Related
I'm starting out w/ NodeJS and Express. Coming from the other popular scripting languages and C++ background, asynchronously calling DB functions is a bit foreign. I've sorted out a pattern, but I'm still curious about catching exceptions. Below is my basic pattern.
var callback = function(req, res) {
// do stuff
connection.query(queryString, function(err,result){
if (err) throw err;
// process results.
};
};
var express = require('express');
var app = express();
app.get('/', callback);
app.listen(3000,function() {
console.log('listening');
};
Generally I have a lot of endpoints and callbacks. I'm a bit lost on where I set up ta try/catch block to catch errors thrown in the callback though. I've looked around for some suggestions, but a lot of them seem to be on the web framework (if any) being used.
When you throw in an asynchronous callback, the exception just goes back to the internals of the database event handler and there is NO way for you to ever catch or handle that exception. So, basically it does no good at all. It will just cause you to abort the handling of that request and you will never send a response on that request.
Basically, you have several choices for how to handle the error. You can handle it completely right in each endpoint and send some sort of error response.
Send Response right at each point of error
app.get('/', function(req, res) {
// do stuff
connection.query(queryString, function(err,result){
if (err) return res.status(500).send(someErrorResponse);
// process results.
};
});
Forward on to centralized error handler
Or, you can forward the error on to a centralized error handler by calling next(err):
app.get('/', function(req, res, next) {
// do stuff
connection.query(queryString, function(err,result){
// if error, forward it on to our centralized error handler
if (err) return next(err);
// process results.
};
});
// centralized error handler - note how it has four parameters
app.use(function(err, req, res, next) {
// formulate an error response here
console.log(err);
res.status(500).send(someErrorMessage)
});
See Nodejs handle unsupported URLs and request types for more info on the ways to have generalized error handlers in Express.
Use promises to collect errors within each route
If you are using more involved sequences of asynchronous operations where you may have more than one async operation sequenced together, then it does get to be a pain to handle errors at every single async operation. This is where using promises with all your async operations more easily allows all the errors to percolate up to one .catch() statement at the top level of each route. You don't say what database you're using, but here's an idea what that looks like. The general idea is that you can write your code so that all promise rejections (e.g. errors) will propagate up to one central .catch() in each route handler and you can then call next(err) from that .catch(), sending the error to your centralized error handler. Here's how that looks for a recent version of Mongoose (you didn't say which database you were using) with one database operation.
app.get('/', function(req, res, next) {
// do stuff
connection.query(queryString).exec().then(function(result){
// process results.
}).catch(next);
});
// centralized error handler - note how it has four parameters
app.use(function(err, req, res, next) {
// formulate an error response here
console.log(err);
res.status(500).send(someErrorMessage)
});
And, here's what it looks like if you have more than one operation:
app.get('/', function(req, res, next) {
// do stuff
connection.query(queryString).exec().then(function(result){
// process results, then make another query
// return the promise from this second operaton so both results
// and error are chained to the first promise
return connection.query(...).exec();
}).then(function(result) {
// process chained result
}).catch(next);
});
// centralized error handler - note how it has four parameters
app.use(function(err, req, res, next) {
// formulate an error response here
console.log(err);
res.status(500).send(someErrorMessage)
});
Since ES6 built in support for promises and ES7 will add support for async/await for asynchronous operations (which is based on promises) and all significant libraries that offer asynchronous operations have added or are adding support for promises, it is clear that promises are the future of the language for managing asynchronous operations. That would be my strong recommendation.
You should never, ever throw an error like that! :) The reason is that at some point your whole node app will just stop working, because of some db query failed. This should be handled instead of just die.
And because this is a route handler - handles specific url that the user is getting (for example /), there should be some output. You can always show a page with status 500 and a nice design, if there was such an error that you cannot handle or you might have your internal state messed up.
So basically just act as nothing happened - return respones of any kind, or even render a page, but provide information that something went wrong.
Also, a common scenario is something like what Alon Oz presented. All routes in express are actually a middleware functions, that are called one after another. If the route does not match the requested one, the function just skips and the next one is called. You can manually control that. The actual pattern of the router is this:
app.get('/', function(req, res, next) {
// you can have the request
// you can send response like res.send('hello')
// OR you can skip this function using NEXT
});
The actual signature of next is next(err). So if you call it without any arguments, it will just skip to the next middleware. If you call it with an argument, it will skip all regular functions and go to the last ones in the stack, or more specifically the ones that handle errors. They are like the regular ones, but taking four arguments instead of three:
app.use(function (err, req, res, next) { });
It's very important to understand that this function will be called if you call next with an argument. Throwing an error won't do any good! Of course if none of your routes match the specific criteria (url) the final one will in the call will be called, so you can still handle the "not found" error.
This is a common scenario that you will use:
// development error handler, will print stacktrace
if (app.get('env') === 'development') {
app.use(function(err, req, res, next) {
debug('ERROR [ip: %s]:: dev env -> ', req.ip, err); // I'm using debug library - very helpful
res.status(err.status || 500);
res.render('deverr', { // I render custom template with the whole stack beautifully displayed
errMessage: err.message,
error: err
});
});
}
// production error handler, no stacktraces leaked to user
app.use(function(err, req, res, next) {
res.status(err.status || 500);
res.render('pages/error', { // custom error page with nice design and a message
errMessage: err.message,
error: {}
});
});
Hope that helps! :)
Since you are using express, it has its own way to handle exceptions,
defined like this:
function clientErrorHandler (err, req, res, next) {
if (req.xhr) {
res.status(500).send({ error: 'Something failed!' })
} else {
next(err)
}
}
app.use(clientErrorHandler)
For more info:
https://expressjs.com/en/guide/error-handling.html
There are most commonly three major types of errors that we need to take into account.
Promise failures (Any failures that come up during async/await or result of a promise in then/catch)
In order to handle promise failures, as suggested in the strong loop document or node js 2018 best practices, it's important to have a common function that can handle it.
// app.js file
app.get('/:id', async (req,res,next) => {
if(!req.params.id) {
return res.status(412).send('enter a valid user id');
}
try {
const results = await UserDAL(id);
} catch(e) {
next(e);
}
}
// common error middleware defined in middleware/error.js
module.exports = function (err,req,res,next) {
logger.error(`${err.status || 500} - ${err.message} - ${req.originalUrl} - ${req.method} - ${req.ip}`);
return res.status(500).send('something failed.');
};
Unhandled Rejections
process.on('unhandledRejection', e => {
// do something
});
Unhandled exceptions
process.on('uncaughtException', e => {
// do something
});
If you see a lot of try/ catch blocks in your express methods you can abstract that to a separate async function like below:
module.exports = function asyncMiddleWare(handler) {
return async (req,res,next) => {
try {
await handler(req,res)
} catch(e) {
next(e);
}
}
};
I'm curious if there's a good way to handle this with express 4.0.
There are times where there is a problem with either Stripe, or my connection to Stripe that needs to be addressed. However, I obviously do not want users to know about this. I want to display a message 'There was a problem completing your order, please contact support.' while safely logging the message with some information for me to handle it.
I suspect I can do this in middleware. However, I'm not sure how. I would like to catch these errors as they are happening and dump them to a logfile of some kind (suggestions would be great) so I can handle it.
Is there a standard way of doing this? How would I accomplish this?
Thank you!
You're thinking about this the wrong way. Instead of exposing 3rd party errors, explicitly define each possible failure case in your route.
// bad
app.use(function (err, req, res, next) {
res.status(err.status).json({ error: err.message })
})
Instead mark errors to be exposed:
app.use(function (err, req, res, next) {
if (err.expose) {
res.status(err.status).json({ code: err.code, error: err.message })
} else {
reportError(err, req)
res.status(500).json({ code: "unknown", error: "unknown error" })
}
})
You need to re-throw actual api errors.
function createError(status, message) {
var err = new Error(message)
err.expose = true
err.status = status
return err
}
function changeCard(user, data) {
return postStripeCards(user, data)
.catch({ type: "card_error" }, function (err) {
throw createError(400, "invalid card")
})
}
Uncaught errors are programmer mistakes which should be reported and fixed. Anything that makes it through is a bug.
function reportError (err, req) {
// log error
console.error(err.stack)
// send to rollbar
rollbar.handleError(err, req)
// maybe send an email
mailgun.send({
from: "bot#myapp.com",
to: "me#gmail.com",
subject: "My App: " + err.message,
text: err.stack + "\n\n" + JSON.stringify(req),
})
// pager duty, airbrake, etc
}
It takes work, but it means your API is well-defined.
In nodejs express to handle exceptions , check for err in callbacks as :
if(err!==null){
next(new Error ('Erro Message'));
}
Which in turn calls the express's error handler middleware .
app.use(function(err, req, res, next){
if(!err) return next();
console.log('<-------Error Occured ----->');
res.send(500, JSON.stringify(err, ['stack', 'message']));
});
But to call invoke next(err) , I'm forced to pass around the reference for next across all the callback methods through all the layers . I find this a messy aproach . Is there a better way to handle exceptions and send a proper response using events or domains .
You should always delegate the error in the routes / controllers to the error handler by calling next (so you can just deal with them in one place instead of having them scattered throughout your application).
Here's an example:
app.get('/', function(req, res, next) {
db.findUser(req.params.userId, function(err, uid) {
if (err) { return next(err); }
/* ... */
});
});
/* Your custom error handler */
app.use(function(err, req, res, next) {
// always log the error here
// send different response based on content type
res.format({
'text/plain': function(){
res.status(500).send('500 - Internal Server Error');
},
'text/html': function(){
res.status(500).send('<h1>Internal Server Error</h1>');
},
'application/json': function(){
res.send({ error: 'internal_error' });
}
});
});
Note: You don't have to check for the err param in the error handler because it will always be present.
Also very important: always do return next(err); because you don't want the success code to be executed.
Both your code samples were flawed: in the first one you didn't use return next(err) and in the second one you have used return next(err), so code that followed shouldn't be handling the error (because it will never get there in case there's an error), but instead it should have been the 'success' code.
The error pages example from Express showsthe canonical way of handling errors:
https://github.com/visionmedia/express/blob/master/examples/error-pages/index.js
// error-handling middleware, take the same form
// as regular middleware, however they require an
// arity of 4, aka the signature (err, req, res, next).
// when connect has an error, it will invoke ONLY error-handling
// middleware.
// If we were to next() here any remaining non-error-handling
// middleware would then be executed, or if we next(err) to
// continue passing the error, only error-handling middleware
// would remain being executed, however here
// we simply respond with an error page.
I'm using a custom error page in Express, as explained here.
But when I do that, I only see the error message. I'd like to get hold of the same information as displayed in the default Express error handler (the stacktrace, etc.), so that I can:
Log it to the console (if I could leave the default in place for this, I'd be happy).
Display it on the error page, but only for localhost.
How do I do this?
This is a modified version from #generalhenry's answer. You can access the stack trace
in err.stack so you can pass it on your '500' view and do some fancy css styles on it.
app.use(function(err, req, res, next) {
if (err instanceof NotFound) {
res.render('errors/404');
} else {
res.render('errors/500', {error: err, stack: err.stack});
}
});
function NotFound() {
this.name = "NotFound";
Error.call(this, msg);
Error.captureStackTrace(this, arguments.callee);
}
// below all route handlers
// If all fails, hit em with the 404
app.all('*', function(req, res){
throw new NotFound;
});
Just use the error provided to the middleware
// Handle 500
app.use(function(error, req, res, next) {
console.error(error);
if (ISLOCALHOST()) {
res.json(error, 500);
} else {
res.send('500: Internal Server Error', 500);
}
});
I have a working node.js / express based server and am using jade for templating. Usually there is no problem but a couple of times every day I get an error message when requsting any page. The error is 'failed to locate view'. I don't know why i get this error since it worked fine just minutes before.
The question however is how I can force a crash on this event, for example:
res.render('index.jade', {info: 'msg'}, function(error, ok) {
if (error)
throw new Error('');
// Proceed with response
};
How would I do this? And how would I proceed with the response?
thank you.
You can add an error handling middleware.
app.use(function handleJadeErrors(err, req, res, next) {
// identify the errors you care about
if (err.message === 'failed to locate view') {
// do something sensible, such as logging and then crashing
// or returning something more useful to the client
} else {
// just pass it on to other error middleware
next(err);
}
});
Try this:
app.use(function (req, res, next) {
fs.exists(__dirname + '/views/' + req.url.substring(1) + '.jade', function (exists) {
if(!exists) {
console.log(err);
return next();
}
res.render(req.url.substring(1), { title: "No Controller", user: req.session.user });
}
});