Develop Reference

Express Try and Catch in Form of middleware

i'm working on node.js using Express to built a backend.
i'm intended to handle status 500 error that may happened.
router.put('/test', async (req, res) => {
try {
return res.send(await request.updateTest(req.body, 1))
} catch(err) {
console.log(err)
return res.status(500).send(err.stack)
}
})
this is my example of the code. it's do work perfectly. but when i'm try to make unknown error from the database query, i want to log the error and return status 500 as response with the error detail.
but i'll need to add try and catch every time i'm build a new controller/routes
is there anyway i could express them in form of middleware instead of write try and catch everytime?
this is an example of code i've try to make it as middleware but it's has no work and no effect when called.
error.js
module.exports = function (err, req, res, next) {
console.log(err)
res.status(500).send({
error: 'Internal Server Error',
message: err.stack
})
next(err)
}
main.js
const errorHandler = require('./error')
const { corsOption } = require('./cors')
const cors = require('cors')
const test = require('./test')
module.exports = function (app) {
app.use(cors(corsOption))
app.use(errorHandler)
app.use('/api/test', test)
}
is there anyway that i can do for this to work?

Your global error handler should be placed after all other middlewares/routes:
app.use(middleware)
// all other middlewares
app.use('/api/test', test)
// all other routes
// error handler
app.use(function (err, req, res, next) {
res.status(500).json({
error: err.message,
});
});
To avoid adding try/catch to everything, better to wrap your route handler to catch the errors (asyncWrapper):
app.use(middleware)
// all other middlewares
const asyncWrapper = (cb) => {
return (req, res, next) => cb(req, res, next).catch(next);
};
const test = async (req, res) => {
return res.send(await request.updateTest(req.body, 1))
}
// wrap your handler to catch errors (Async functions return a promise)
app.use('/api/test', asyncWrapper(test))
// all other routes
// error handler
app.use(function (err, req, res, next) {
res.status(500).json({
error: err.message,
});
});

There are two approaches to resolve unhandled exceptions in Node.js
Using try-catch blockwhich is already you are using
Using Process i.e use Process to handle exception. A process is a global object that provides information about the current Node.js process. The process is a listener function that is always listening to the events. The most effective and efficient approach is to use Process. If any uncaught or unhandled exception occurs in your code flow, that exception will be caught in code
process.on('uncaughtException', function(err) {
// Handle the error safely
console.log(err)
})
The above code will be able to handle any sort of unhandled exception which occurs in Node.js. see this Process Events

ExpressJS doesn't wait for my promise

I'm making a search-page on my server. When the endpoint is reached and the user waits for the search function to return the results and render the page Express falls through to the 404 handler instead, and I get the following error when I suppose the render function is called:
Error: Can't set headers after they are sent.
What am I doing wrong?
router.get("/", async (req, res) => {
try {
const queryString = req.query.q;
const user = helper.checkAndGetUser(req, res);
let s = String(queryString), searchedTags = [""];
if(s.indexOf(",") > -1){
searchedTags = s.replace(" ", "").split(",");
}
const options = {
"query": {tags: {$all: searchedTags}, _forSale: true}
};
const results = await Search.search(options).then(result => result).catch(err => {
throw err;
});
//This res.render -call is called after the 404 splat-route.
return res.render("partial/search.pug", {user: user, search: {
query: queryString,
results: results
}});
//If I'd use res.send for debugging, it is instead called before the splat-route, like the following:
return res.send(results);
} catch(err) {
next(err);
}
});
module.exports = router;
I register the router:
const search = require("./search.js");
app.use("/search", search);
Followed by the 404 splat-route:
app.get("*", async (req, res, next) => {
const user = helper.checkAndGetUser(req, res);
res.status(404);
res.render("partial/404.pug", {user: user});
});
To clarify:
My question is how can I make the res.render function get called just as the res.send function?
UPDATE [2017-10-05]:
I continued with another part of the site, a similar endpoint, and discovered that sending the result provided by the promise worked as expected if using res.send but not res.render. Using res.render the 404-handler kicked in again. Can this be a bug in Express?

This happens if you attempt to write to res after it is sent, so you must be calling additional code after res.render() or you already responded before calling that.
change it to return res.render(...) so it exits the functions, otherwise it will continue through the function and hit other res.render()s etc.
Something is up with that error handler also. I will update my post in a few mins with tips (on phone). It should probably have (req, res, next) and call return next(err) and pass it to your error handling middleware.
Here is the pattern I like to use in async/await Express:
// these routes occur in the order I show them
app.get('/route', async (req, res, next) => {
try {
const data = 'asdf'
const payload = await something(data)
.then((result) => createPayload(result))
// remember, if you throw anywhere in try block, it will send to catch block
// const something = willFail().catch((error) => {
// throw 'Custom error message:' + error.message
// })
// return from the route so nothing else is fired
return res.render('route', { payload })
} catch (e) {
// fire down to error middleware
return next(e)
}
})
// SPLAT
app.get('*', async (req, res, next) => {
// if no matching routes, return 404
return res.status(404).render('error/404')
})
// ERRORS
app.use(async (err, req, res, next) => {
// if err !== null, this middleware fires
// it has a 4th input param "err"
res.status(500).render('error/500')
// and do whatever else after...
throw err
})
Note: next() callback called without param is treated as no error, and proceeds to the next middleware. If anything is passed in, it will fire the error middleware with the param as the value of err in the error handling middleware. You can use this technique in routes and other middlewares, as long as the error middleware comes last. Mind your use of return with res.send/render() to prevent double setting headers.
NEW:
Something looks a little bit off with that .then() having a callback in it. I don't see logically where err would come from since the value of the resolved promise goes into the .then() function as result. At this point, it is suspect and should be removed or refactored if possible. This part here:
try {
let results = [];
await Search.search(options).then(result => {
results = result;
}, err => {
throw err;
});
console.log("res.render");
return res.render("partial/search.pug", {user: user, search: {
query: string,
results: results
}});
} catch(err) {
next(err);
}
First, here is about what I would expect to see with async/await syntax:
router.get("/", async (req, res, next) => {
try {
const queryString = req.query.q;
const user = helper.checkAndGetUser(req, res);
let s = String(queryString), searchedTags = [""];
if (s.indexOf(",") > -1) {
searchedTags = s.replace(" ", "").split(",");
}
const options = {
"query": { tags: { $all: searchedTags }, _forSale: true }
};
// If a promise is ever rejected inside a try block,
// it passes the error to the catch block.
// If you handle it properly there, you avoid unhandled promise rejections.
// Since, we have async in the route function, we can use await
// we assign the value of Search.search(options) to results.
// It will not proceed to the render statement
// until the entire promise chain is resolved.
// hence, then(data => { return data }) energizes `results`
const results = await Search.search(options)
.then(data => data)
// If any promise in this chain is rejected, this will fire
// and it will throw the error to the catch block
// and your catch block should pass it through to your
// error handling middleware
.catch(err => { throw 'Problem occurred in index route:' + err });
return res.render("partial/search.pug", {
user: user, search: {
query: string,
results: results
}
});
} catch (err) {
// look at the top how we added next as the 3rd, callback parameter
return next(err);
}
});
module.exports = router;
Error handler:
// notice how we add `err` as first parameter
app.use((err, req, res, next) => {
const user = helper.checkAndGetUser(req, res);
res.status(404);
res.render("partial/404.pug", {user: user});
});
From the Express docs:
Define error-handling middleware functions in the same way as other middleware functions, except error-handling functions have four arguments instead of three: (err, req, res, next). For example:
app.use(function (err, req, res, next) {
console.error(err.stack)
res.status(500).send('Something broke!')
})
http://expressjs.com/en/guide/error-handling.html
That might be your true issue because the error handler should only fire if next() is called with any input, but yours appears to be firing every time like a normal middleware, so I suspect it's because there is no err parameter on that middleware function, so it is treated as a normal one.
The Default Error Handler
Express comes with a built-in error handler, which takes care of any errors that might be encountered in the app. This default error-handling middleware function is added at the end of the middleware function stack.
If you pass an error to next() and you do not handle it in an error handler, it will be handled by the built-in error handler; the error will be written to the client with the stack trace. The stack trace is not included in the production environment.
If you call next() with an error after you have started writing the response (for example, if you encounter an error while streaming the response to the client) the Express default error handler closes the connection and fails the request.
So when you add a custom error handler, you will want to delegate to the default error handling mechanisms in Express, when the headers have already been sent to the client:
// code example in docs
Note that the default error handler can get triggered if you call next() with an error in your code more than once, even if custom error handling middleware is in place.
I also recommend using that splat route app.get('*', async (req, res, next) => {}) right above the error handler middlware (aka as the last loaded route in your list). This will catch all unmatched routes, such as /sih8df7h6so8d7f and forward the client to your 404. I think the error handler middlware is more suited for error 500 and clean formatted type errors because it gives you a function that can parse the value of next(err) anytime it is called from a route.
I usually do this for authentication failures with JSON web token (as the first line of code inside every auth required route):
if (!req.person) return res.status(403).render('error/403')
I realize some of this may fry your wig wholesale, so try all this stuff out and see each piece working before you determine if you would like to utilize it or not.

After a few days of going through the code again and again I stumbled upon a problem in the checkAndGetUser-function, that when run without the user being signed in, and since it's faster than the async call to the DB, triggered the splat endpoint and thus showed the 404 page.
I believe the reason for not triggering the splat endpoint when the res.render call was replaced by res.send was that the res.send function is that much faster than the render-call, since it doesn't have to parse any HTML.
Thank you #agm1984 for providing very useful information about the Express framework, if anyone else are having the same or a similar problem, make sure to read through his post.

Some code confusing in MEAN Web Development 2en

exports.signin = function(req, res, next) {
passport.authenticate('local', function(err, user, info) {
if (err || !user) {
res.status(400).send(info);
} else {
// Remove sensitive data before user.password = undefined; user.salt = undefined;
req.login(user, function(err) {
if (err) {
res.status(400).send(err);
} else {
res.json(user);
}
});
}
})(req, res, next);
};
This a piece of code in the "MEAN Web Development" book by AmosQ.Haviv.Who could tell me what the method passport.authenticate()'tail:(req, res, next) means?Is that a Closure?

passport.authenticate() will probably take in the settings 'local' (a domain ? ) and the calllback to create a new function that will do the authentication.
This function will indeed create a closure over 'local' and the callback.
The new auth function will expect 3 parameters: the original req(uest), res(ponse) object and a next parameter and is immediately called using the ( req, res, next ) syntax.
So it's very likely that this specific function will do the login and then run the callback used to create the auth function, passing req and res back into the callback.
Look at it as a way to use the passport.authenticate() method to create different versions of logins you can use. One for 'local', one for 'otherDomain', etc.

Async wrap function for express 4 doesn't catch error

I am trying out this trick from strongloop https://strongloop.com/strongblog/async-error-handling-expressjs-es7-promises-generators/ for having a wrap function for async routes
but the error function is never called. I also tried to put the error function inside the authRouter file.
in authRouter.js:
let wrap = fn => (...args) => fn(...args).catch(args[2]);
router.post('/login', wrap(async (req,res) => {
if (!req.body.email || !req.body.password) throw new Errors.BadRequestError();
}));
export default router;
and in app.js
app.use('/auth', authRouter);
app.use(function(err, req, res) {
console.log('in here');
const status = err.status || 500;
if (status === 500) console.log(err);
res.status(status);
res.send({
message: err.message,
error: err
});
});

You need to have 4 parameters in the error handler to make express recognise it as one:
(from http://expressjs.com/en/guide/error-handling.html): "Define error-handling middleware functions in the same way as other middleware functions, except error-handling functions have four arguments instead of three: (err, req, res, next)."
this is the last error handler I don't wanna call next()
That doesn't really matter, you still have to declare it even if you don't use it.

node.js + Express - How to properly throw/catch errors

I just started a project using Express and I have no real knowledge in node.js, and even in Javascript in general. I was wondering what was the proper way of throwing and catching errors so that they display correctly, and also so that I don't duplicate too much code. I thought this bit of code in app.js:
// catch 404 and forward to error handler
app.use(function(req, res, next) {
var err = new Error('Not Found');
err.status = 404;
next(err);
});
// error handlers
// development error handler
// will print stacktrace
if (app.get('env') === 'development') {
app.use(function(err, req, res, next) {
res.status(err.status || 500);
res.render('error', {
message: 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('error', {
message: err.message,
error: {}
});
});
acted like a big catch that would intercept all the errors I would throw in my controllers. But I don't really know how to make it work. For example, let's take this bit of code in users.js:
/* GET users listing. */
router.get('/', function(req, res, next) {
connection.query('SELECT id, name FROM user', function(err, rows, fields) {
if (err) throw err;
res.render('users', {
title: 'Users',
userList: rows
});
});
});
How do I send something like a 204 code (No Content)
How do I report an error with the database
Any help/hint would be appreciated, I'll keep looking on forums and stuff in the meantime.

Express uses a series of middleware to handle the request. If the route matches and defined route it will handle it. Otherwise, you can put '/404' route at last after importing all the routes. It will automatically reach there when no route is found. Else if you don't want to send a response you can use next() so that it moves to next middleware and ultimately to '/404' middleware.
/* GET users listing. */
router.get('/', function(req, res, next) {
connection.query('SELECT id, name FROM user', function(err, rows, fields) {
if (err) {
return res.status(502).json({message:'db error'});
};
res.render('users', {
title: 'Users',
userList: rows
});
});
});

Thank you all for your answers. What I was looking for (and found) was the next(error) function to pass control to the error handler located in app.js (Cf. http://expressjs.com/en/guide/routing.html for more details).

You can give a relevant message with statusCode like this
/* GET users listing. */
router.get('/', function(req, res, next) {
connection.query('SELECT id, name FROM user', function(err, rows, fields) {
if (err){
res.status(500).send("Something is not right");
};
//if you get rows of array type, then you can do this for empty records
else if(rows.length===0){
res.status(204).send('No records found!')
}else{
res.render('users', {
title: 'Users',
userList: rows
});
}
});
});

In Express, each route handler is passed a 'res' parameter, that is used to provide the response to the client. Sample usage of creating a response with a specific code, code error and empty body
router.get('/', function(req, res, next) {
res.status(401, "Authentication mismatch").json({});
});
Instead of json() you can also use send() to send a plain text back.