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When to use next() and return next() in Node.js
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Scenario: Consider the following is the part of code from a node web app.
app.get(‘/users/:id?’, function(req, res, next){
var id = req.params.id;
// do something
next(); //or return next();
Issue: I am checking which one to go with just next() or return next(). Above sample code works exactly the same for both & did not show any difference in execution.
Question: Can some one put light on this, when to use next() and when to use return next() and some important difference?
node.js express connect v8
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edited Mar 5 ’18 at 4:38
Amol M Kulkarni
asked May 29 ’13 at 9:39
Amol M KulkarniAmol M Kulkarni
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As @ ‘s answer:
If you don’t do it, you risk triggering the callback a second time later, which usually has devastating results
I give an example here if you write middleware like this:
app.use((req, res, next) => {
console.log(‘This is a middleware’)
console.log(‘This is first-half middleware’)
app.use((req, res, next) => {
console.log(‘This is second middleware’)
app.use((req, res, next) => {
console.log(‘This is third middleware’)
You will find out that the output in console is:
This is a middleware
This is second middleware
This is third middleware
This is first-half middleware
That is, it runs the code below next() after all middleware function finished.
However, if you use return next(), it will jump out the callback immediately and the code below return next() in the callback will be unreachable.
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answered Apr 14 ’17 at 2:01
PJCHENderPJCHENder
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As a beginner to express this answer made things clearer to me than the other answers. Thumbs up!
– mandarin
Dec 19 ’17 at 13:44
Would a similar thing be true of res.redirect(‘/’) vs. return res.redirect(‘/’) in this type of situation? Maybe it’s just better to always write return in front of res statements to avoid errors of setting headers after they were sent?
Sep 7 ’18 at 6:45
Why should I write code after next()? Isn’t it obvious that I do nothing after I finished my task in a middleware? @PJCHENder
Jul 25 ’19 at 6:17
@ImranPollob sometimes mistakes happens. When you write a lot of code, ifs/elses/etc. You may forget “`return next()`
Jul 25 ’19 at 16:45
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Some people always write return next() is to ensure that the execution stops after triggering the callback.
If you don’t do it, you risk triggering the callback a second time later, which usually has devastating results. Your code is fine as it is, but I would rewrite it as:
app.get(‘/users/:id?’, function(req, res, next){
var id = req.params.id;
return next();
// do something
It saves me an indentation level, and when I read the code again later, I’m sure there is no way next is called twice.
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edited Apr 30 ’17 at 13:01
user6451184
answered May 29 ’13 at 10:53
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Would a similar thing be true of res.redirect(‘/’) vs. return res.redirect(‘/’) in this type of situation? Maybe it’s just better to always write return in front of res statements to avoid errors of setting headers after they were sent?
Sep 7 ’18 at 6:46
@AdamD I would like to know that as well.
– theprogrammer
Sep 1 ’20 at 23:01
@theprogrammer Yes, I think this answer applies the same way to things like res.redirect and it would be best practice to use it, unless you have some other server tasks you want to do after redirecting the user.
Sep 4 ’20 at 1:47
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next() is part of connect middleware. Callbacks for router flow doesn’t care if you return anything from your functions, so return next() and next(); return; is basically the same.
In case you want to stop the flow of functions you can use next(err) like the following
app.get(‘/user/:id?’,
function(req, res, next) {
console.log(‘function one’);
if ( !req.params.id )
next(‘No ID’); // This will return error
next(); // This will continue to function 2
function(req, res) {
console.log(‘function two’);
Pretty much next() is used for extending the middleware of your requests.
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edited May 1 ’17 at 10:07
answered May 29 ’13 at 11:01
drinchevdrinchev
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Can we send parameter like: next(‘No ID’) ?
– Amol M Kulkarni
May 29 ’13 at 12:10
next(‘No ID’) is actually sending an error, which will break the flow.
– drinchev
May 29 ’13 at 12:15
Use next(null, “somevalue”); For tools like async.waterfall it will pass the value to the next function. For complex series of interactions that are data driven, I usually pass a context object between functions. That way I can create generic functions that can be shared across multiple end points and control flow via data in the context
– Chad Wilson
Oct 30 ’15 at 0:12
“so return next() and next(); return; is basically the same.” – just what I needed to read. thx @drinchev
Feb 25 ’16 at 1:25
I observe the opposite (when firing error): next(error) triggers next middleware, but continues to execute code; return next(error) just relegates execution to the next middleware. next(e) and return next(e) are NOT the same.
– Nickolodeon
Jul 25 ’18 at 17:06
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