How to properly handle errors in Express?
Solution 1
You'll want to check out Express Error Handling. From there:
app.param('userId', function(req, res, next, id) {
User.get(id, function(err, user) {
if (err) return next(err);
if (!user) return next(new Error('failed to find user'));
req.user = user;
next();
});
});
The sweetspot that you are missing is the return
next(...)
Solution 2
That's because you're doing it wrong: you already threw an Error (which will be processed by Express and return a 500 - Error page for the user or something like that) but you are also trying to send your own response to the client: res.send('event found!');
You should really check out the Express guide about Error Handling here: http://expressjs.com/guide/error-handling.html
What I would do in your example is:
function NotFound(msg){
this.name = 'NotFound';
Error.call(this, msg);
Error.captureStackTrace(this, arguments.callee);
}
app.get('/event/:id', function(req, res, next){
if (req.params.id != 1) {
throw new NotFound('Cannot find event ' + req.params.id);
} else {
res.send('event found!');
}
});
app.error(function(err, req, res, next){
if (err instanceof NotFound) {
res.render('404.ejs');
} else {
next(err);
}
});
Solution 3
You have a couple of problems in your code:
When responding to the client, you need to use the response object (
res
rather thanreq
).When sending an error to
next
, you should return, so the rest of the function doesn't run.
Here's your code after fixing those errors:
app.get('/event/:id', function(req, res, next) {
if (req.params.id != 1) {
return next(new Error('cannot find event ' + req.params.id));
}
res.send('event found!'); // use res.send (NOT req.send)
});
YWCA Hello
Updated on July 09, 2022Comments
-
YWCA Hello almost 2 years
I am beginning to work with Express JS and have run into an issue. I can't seem to figure out the proper way to handle errors.
For example, I have a web services API that serves an object called "event". I'd like to return a simple string of "cannot find event" when a user submits an event id that isn't found. Here is how I'm currently structuring my code:
app.get('/event/:id', function(req, res, next) { if (req.params.id != 1) { next(new Error('cannot find event ' + req.params.id)); } req.send('event found!'); });
When I submit an id other than 1, Node crashes with the following output:
http.js:527 throw new Error("Can't set headers after they are sent."); ^ Error: Can't set headers after they are sent. at ServerResponse.<anonymous> (http.js:527:11) at ServerResponse.setHeader (/usr/local/kayak/node_modules/express/node_modules/connect/lib/patch.js:62:20) at /usr/local/kayak/node_modules/express/node_modules/connect/lib/middleware/errorHandler.js:72:19 at [object Object].<anonymous> (fs.js:107:5) at [object Object].emit (events.js:61:17) at afterRead (fs.js:878:12) at wrapper (fs.js:245:17)
From what I can tell by using the node.js debugger, execution of the block of code continues after
next()
is called, meaning thatreq.send('event found!')
tries to run. I don't want this to happen.The only workaround that I've found is to simply throw a
new Error()
instead of "next-ing" it, but this results in a default Express HTML error page being generated. I'd like a little more control than that.I have taken the time to read over the error handling section of the Express documentation, but I couldn't make sense of it.