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);
}
}
};
Related
I'm quite new to NodeJS, and I came across a construction that I can't wrap my head around. Consider the following code:
router.get("/", function(req, res, next) {
let db = new sqlite3.Database(dbname, sqlite3.OPEN_READONLY, (err) => {
if (err) next(err);
});
res.render("views/page");
}
If an exception is raised by Database (for instance because dbname does not exist), then the callback will pass it to the Express error handler (next). But the code does not stop there, and it will continue to execute the next line and attempt to render something, which is problematic if the error handler also sends headers. If I add a return statement within the callback, it will simply terminate Database but not the rest of the function.
My question: is there anyway to prevent the rest of the code from being executed if an exception is raised?
Please read the sqlite3 npm documentation. Ideally, you should not be opening a database connection on every request. Instead, you should open the database connection when the server is started, and share a reference to the db variable to be used in the API routes (gets/posts).
Unless you just want to make it work, I believe the below should do it.
router.get("/", function(req, res, next) {
let db = new sqlite3.Database(dbname, sqlite3.OPEN_READONLY, (err) => {
if (err) return next(err);
res.render("views/page");
});
}
EDIT: Javascript does not allow for async/await when instantiating objects, so in this case I believe would be better to just initialize the database connection outside the router method, like below, and use the error event to handle errors
const db = new sqlite3.Database(dbname, sqlite3.OPEN_READONLY);
db.on('error', function(err) { /*handle error here*/ });
router.get("/", function(req, res, next) {
res.render("views/page");
});
}
These two middleware functions behave differently and I cannot figure out why:
Here, the error will get trapped by try/catch:
router.get('/force_async_error/0', async function (req, res, next) {
try{
await Promise.reject(new Error('my zoom 0'));
}
catch(err){
next(err);
}
});
But here, the error will not get trapped by try/catch:
router.get('/force_async_error/1', async function (req, res, next) {
await Promise.reject(new Error('my zoom 1'));
});
I thought Express wrapped all middleware functions with try/catch, so I don't see how it would behave differently?
I looked into the Express source, and the handler looks like:
Layer.prototype.handle_request = function handle(req, res, next) {
var fn = this.handle;
if (fn.length > 3) {
// not a standard request handler
return next();
}
try {
fn(req, res, next); // shouldn't this trap the async/await error?
} catch (err) {
next(err);
}
};
so why doesn't the try/catch there capture the thrown error?
I'm going to add an answer here even though you've already accepted another one because I think what's going on here can be explained better and this will help others attempting to understand this.
In your code here:
router.get('/force_async_error/1', async function (req, res, next) {
await Promise.reject(new Error('my zoom 1'));
});
Let's discuss what is going on:
First, you declared the callback as async which you had to do in order to use await in it. An async function tells the interpreter to do several important things.
1. An async function always returns a promise. The resolved value of the promise will be whatever the function returns.
2. An async function is internally wrapped with a try/catch. If any exceptions are thrown in the top level scope of the function code, then those exceptions will be caught and will automatically reject the promise that the function returns.
3. An async function allows you to use await. This is an indicator to the interpreter that it should implement and allow the await syntax inside the function. This is tied to the previous two points above which is why you can't use await in just any 'ol function. Any uncaught rejections from await will also reject the promise that the function returns.
It's important to understand that while the async/await syntax allows you to kind of program with exceptions and try/catch like synchronous code, it isn't exactly the same thing. The function is still returning a promise immediately and uncaught exceptions in the function cause that promise to get rejected at some time later. They don't cause a synchronous exception to bubble up to the caller. So, the Express try/catch won't see a synchronous exception.
But here, the error will not get trapped by try/catch
I thought Express wrapped all middleware functions with try/catch, so I don't see how it would behave differently?
so why doesn't the try/catch [in Express] there capture the thrown error?
This is for two reasons:
The rejected promise is not a synchronous throw so there's no way for Express to catch it with a try/catch. The function just returns a rejected promise.
Express is not looking at the return value of the route handler callback at all (you can see that in the Express code you show). So, the fact that your async function returns a promise which is later rejected is just completely ignored by Express. It just does this fn(req, res, next); and does not pay attention to the returned promise. Thus the rejection of the promise falls on deaf ears.
There is a somewhat Express-like framework called Koa that uses promises a lot and does pay attention to returned promises and which would see your rejected promise. But, that's not what Express does.
If you wanted some Koa-type functionality in Express, you could implement it yourself. In order to leave other functionality undisturbed so it can work normally, I'll implement a new method called getAsync that does use promises:
router.getAsync = function(...args) {
let fn = args.pop();
// replace route with our own route wrapper
args.push(function(req, res, next) {
let p = fn(req, res, next);
// if it looks like a promise was returned here
if (p && typeof p.catch === "function") {
p.catch(err => {
next(err);
});
}
});
return router.get(...args);
}
You could then do this:
router.getAsync('/force_async_error/1', async function (req, res, next) {
await Promise.reject(new Error('my zoom 1'));
});
And, it would properly call next(err) with your error.
Or, your code could even just be this:
router.getAsync('/force_async_error/1', function (req, res, next) {
return Promise.reject(new Error('my zoom 1'));
});
P.S. In a full implementation, you'd probably make async versions of a bunch of the verbs and you'd implement it for middleware and you'd put it on the router prototype. But, this example is to show you how that could work, not to do a full implementation here.
This is because the call is asynchronous, take this code :
try {
console.log('Before setTimeout')
setTimeout(() => {
throw new Error('Oups')
})
console.log('After setTimeout')
}
catch(err) {
console.log('Caught', err)
}
console.log("Point of non-return, I can't handle anything anymore")
If you run it you should see that the error is triggered after Point of non-return.
When we're at the throw line it's too late, we're outside of try/catch. At this moment if an error is thrown it'll be uncaught.
You can work around this by using async/await in the caller (doesn't matter for the callee), ie :
void async function () {
try {
console.log('Before setTimeout')
await new Promise((resolve, reject) =>
setTimeout(() => {
reject(new Error('Oups'))
})
)
console.log('After setTimeout')
}
catch(err) {
console.log('Caught', err.stack)
}
console.log("Point of non-return, I can't handle anything anymore")
}()
Finally, this means that for Express to handle async errors you would need to change the code to :
async function handle(req, res, next) {
// [...]
try {
await fn(req, res, next); // shouldn't this trap the async/await error?
} catch (err) {
next(err);
}
}
A better workaround:
Define a wrap function like this :
const wrap = fn => (...args) => Promise
.resolve(fn(...args))
.catch(args[2])
And use it like this :
app.get('/', wrap(async () => {
await Promise.reject('It crashes!')
}))
Neither of these really answer the question, which if I understand correctly is:
Since the async/await syntax lets you handle rejected "awaits" with non-async style try/catch syntax, why doesn't a failed "await" get handled by Express' try/catch at the top level and turned into a 500 for you?
I believe the answer is that whatever function in the Express internals that calls you would also have to be declared with "async" and invoke your handler with "await" to enable async-catching try/catch to work at that level.
Wonder if there's a feature request for the Express team? All they'd need to add is two keywords in two places. If success, do nothing, if exception hand off to the error handling stack.
Beware that if you don't await or return the promise, it has nothing to do with express - it just crashes the whole process.
For a general solution for detached promise rejections:
https://stackoverflow.com/a/28709667
Copied from above answer:
process.on("unhandledRejection", function(reason, p){
console.log("Unhandled", reason, p); // log all your errors, "unsuppressing" them.
//throw reason; // optional, in case you want to treat these as errors
});
I'm trying to figure out from the documentation how to properly deal with errors (such as a bad template) in the optional callback to res.render in Express.
I am calling it pretty much identically to the example in the docs and using it in certain situations to append extra data to the rendered output.
res.render('template', undefined, (err, html) => {
if (err) // then what?
var processed = process(html)
res.send(processed)
})
The documentation says:
callback, a callback function. If provided, the method returns both the possible error and rendered string, but does not perform an automated response. When an error occurs, the method invokes next(err) internally.
But it seems like the only way to get proper behavior is if I can next(err) myself within the callback. Can someone tell me what the docs are trying to say here when they say next(err) will be called automatically?
Expressjs has a very good documentation on error handling. So, what you can do is throw an error if err exists:
res.render('template', undefined, (err, html) => {
if (err) throw new Error('Something went wrong in render');
var processed = process(html)
res.send(processed)
});
Then define a middleware that handles your error. You define it last after other app.use() and routes calls. A basic error handling middleware looks something like the following:
app.use(function(err, req, res, next) {
res.status(err.status || 500);
res.render('error', {
message: err.message,
error: err
});
});
It is imperative this middleware is error free. This is because this is the last bit of code that will catch any error that is passed down from the rest of your code defined above it.
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.