So far, I have checked that to handle error for promise in generic way, unhandledRejection event handler is the best way to do it. Just like below:
process.on('unhandledRejection', (reason,promise) => {
console.log('error: unhandledRejection');
});
Now the challenge comes when I want to access Request and Response Objects inside this event handler. So that I can generate the 500 Response in generic way and hence I wont need to add promise exception handling everywhere in the project.
I have been suggested to use:
app.use(function (req, res, next) {
process.on('unhandledRejection', function (reason,promise) {
console.error("Here I can access request objects.")
// Unregister unhandledRejection
});
});
But in above case the event listener will be registered multiple times and will only be unregistered whenever exception occurred. As the event will be registered multiple times, So the code(console.error("Here I can access request objects.")) will be triggered multiple times.
Any suggestions ? If I can access request and response objects out side the app.use?
Thanks in Advance!
By default express does handle the error. By express documentation:
Express comes with an embedded error handler that will take care of any errors that may occur in the application. This default error-handling middleware function is added at the end of the middleware function stack.
next() If you give an error to the function and do not process it in a custom error handler, this error is handled by the embedded error handler; the error is printed on the client with stack-trace. Stack-trace is not included in the production environment.
Set the NODE_ENV value of the environment variable to production to run the application in production mode .
When an error is printed, the following information is added to the response:
res.statusCodethe value of the err.statusfield comes from the field
(or err.statusCode). If this value is not in the range of 4xx or 5xx,
it will be set to 500.
res.statusMessage field is set according to
the status code.
If in production mode, the body (body) becomes the HTML of the
status code message, otherwise err.stack.
err.headers Any header specified in object.
next() If you call the function with an error after starting to write the response (for example, if you encounter an error while transmitting the response to the client) the default Express error handler will close the connection and fail the request.
When you add a custom error handler, you should authorize the default Express error handler if the headers have already been sent to the client:
function errorHandler (err, req, res, next) {
if (res.headersSent) {
return next(err)
}
res.status(500)
res.render('error', { error: err })
}
Calling the next()function multiple times with an error in your code can trigger the default error handler, even if the custom error handler middleware is in place.
This is prone to memory leak:
app.use(function (req, res, next) {
process.on('unhandledRejection',
function (reason,promise) {
console.error("Here I can access request objects.")
// Unregister unhandledRejection
});
});
Also, you can't access the response object in 'unhandledRejection', and why do you want to access that if express can handle that for you to send '500' status code to the user. It is always a good practice to catch promises whenever and wherever required. However, if some promises are still not covered there are libraries to handle it too like : express-promise-router, express-async-errors
We could emit meaningful error response with err argument in express error handler and the response will be sent to the user. for example,
app.use((err, req, res, next) => {
if (err.name === 'CustomError' && err.message === 'EmptyResponse') {
return res.status(404).send('Not Found');
}
// ... more error cases...
return res.status(500).send('Internal Server Error');
});
Related
The following code is working fine, and the client is able to connect only if there is a valid token.
But how can i handle the case of invalid token, i.e. catch error thrown by middleware through next?
Also, is there some way to respond to client?
io.use((socket, next) => {
const token = socket.handshake.headers['authorization'].split(' ');
if (token === undefined || token === null || token[0] !== 'Bearer')
return next(new Error('Invalid Token'));
jwt.verify(token[1], jwtSecretKey, (err, decoded) => {
if (err) return next(new Error('Invalid Token'));
console.log(decoded);
next();
});
});
io.use() // Handle here, if error thrown by previous middleware
io.on('connection', (socket) => {
console.log(socket.id + ' Connected');
});
From the socket.io doc, "errors passed to socket.io middleware callbacks are sent as special error packets to clients".
This means that if you call next(err), then it sends that err object to the client that looks like it will generate the error message back to the client. There does not appear to be the same type of error handler that Express has. So, if you want to handle errors in a common place, then instead of calling next(err), you can just create your own error handling function and call it instead handleErrors(socket, err). Then, you will have all the errors passed to one place and you can decide what to send back to the client.
It seems to me that socket.io messages aren't really analogous to http requests as there's no required response and no standard set of errors. In fact, most socket.io messages that generate an error would require error handling specific to that type of message, not something a generic handler can necessarily be good at. For that reason, I'd probably prefer to just handle the error in the middleware where it first occurs (which may or may not need to send something to the client) and only call some common error handling function if you want some common type of response that you can implement one place.
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 have some error handling in my Express app, for async/await functionality, ie. trying to handle the uncaught errors centrally, to respond with appropriate status codes/messages.
I'm doing like so:
const handleRejection = (res, reason) => {
const { code, message } = reason
console.trace()
logger.error(reason)
// use `res` to send response to client
}
app.use((req, res, next) => {
process.on('unhandledRejection', handleRejection.bind(this, res))
next()
})
process.on('SIGTERM', () => process.removeListener('unhandledRejection', handleRejection))
This works fine for catching/handling errors, however, my logs are filled up each time an error is triggered. I don't believe this event listener, process.on('unhandledRejection'), is being removed correctly at all...
Is there a solution for this?
It seems that you're attaching a new event handler on every request.
Then, on SIGTERM you try to remove an event handler handleRejection which was never attached - you didn't attach handleRejection but handleRejection.bind(this, res) which returns a different function.
It looks like you may also be leaking memory by binding the function to every res object for every request.
This is a very strange way to handle errors. I'm not even sure that this is indeed what you are trying to do. Do you want to add so many event hendlers (one for every request made to your server) and then remove all of them on SIGTERM, when you try to exit your server?
Here is my solution for an Express middleware to pass unhandledRejection to main error middleware
// Handle unhandledRejection and pass error to next middleware
app.use(function (req, res, next) {
function unhandledRejection(reason, p) {
console.error('Possibly Unhandled Rejection at: Promise ', p, " reason: ", reason);
next(reason);
}
process.on('unhandledRejection', unhandledRejection);
// Manage to get information from the response too, just like Connect.logger does:
var end = res.end;
res.end = function (chunk, encoding) {
// Prevent MaxListener on process.events
process.removeListener('unhandledRejection', unhandledRejection);
res.end = end;
res.end(chunk, encoding);
};
next();
});
I'm trying to add Bugsnag to my Node Restify service. We have a ton of routes already and such so I'm trying not to add Bugsnag calls all over our code base and I'm also trying to do something global so there's never a mistake where a dev forgets to add the error reporting.
Conceptually I want after any res.send() to get the status code. If the statusCode is >=400 i want to notify Bugsnag by calling Bugsnag.notify. I already check for errors everywhere so no errors ever show up to the clients (browsers, phones, etc) but they do get sent, for example, res.send(401, { message: 'You dont have permission to do that' }) which I'd like to be able to hook into and pass who tried to do that, what route, etc. Problem is I can't get the after event to fire at all:
server.on('after', function (req, res, route, error) {
console.log('AFTER')
});
I think I misunderstand what after is for. It's at the top of my index.js before any routes or other middleware (server.use) is defined.
My general code structure looks something like:
server.post('/foo', function (req, res, next) {
FooPolicy.create(req, function (err) {
if (err) return res.send(err.code, err.data);
FooController.create(req.params, function (response) {
res.send(response.code, response.data)
next();
});
});
});
FooPolicy == checking permissions
FooController == actually creating the model/data
The issue is that the after event is currently treated like any other handler. That means that if you don't call next in every code path, the after event will never be emitted.
In the meantime, adding a next call will cause your after event handler to fire.
if (err) {
res.send(err.code, err.data);
next();
return;
}
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.