How do I capture a "response end" event in node.js+express?

events node.js express response

30,946

Solution 1

Reading the documentation, here is the signature of res.send:

res.send(body|status[, headers|status[, status]])

Which means you can set your own status, like this: res.send( 'some string', 200 ); or even just res.send( 404 );.

This method is the one you use to send the response.

Once it is sent to the client, you can't access it anymore, so there is no callback.

This is the last thing your server does. Once it has processed the request, it sends the response.

However, you can access it before you send it to the client. Which means you can:

console.log( res );
res.send( datas );

If you want to rollback/commit, you do it when the database's callback is called, not when the response is gone.

Solution 2

Strangely enough, it appears that the response emits a "finish" event when the response is closed: https://web.archive.org/web/20120825112648/http://sambro.is-super-awesome.com/2011/06/27/listening-for-end-of-response-with-nodeexpress-js/

Despite this blog entry being a bit old, this event still exists (Line 836 in lib/http.js), so I assume it won't disappear soon, even though neither node's nor express' documentation mention it. By early 2014 it has moved to line 504 on of lib/_http_outgoing.js and still works.

By the way, the "error" event on a server response is probably not something you'd usually want to see.

Solution 3

Just to further @Amatsuyu's response, here's how it should look:

res.on('finish', function() {
  // do stuff here
});

30,946

Author by

Jake

https://linkedin.com/in/jakemiles

Updated on October 12, 2021

Comments

Jake over 2 years
I'd like to write an express middleware function that sets up a listener on the response's 'end' event, if one exists. The purpose is to do cleanup based on the http response code that the end handler decided to send, e.g. logging the response code and rollback/commit of a db transaction. i.e., I want this cleanup to be transparent to the end caller.

I'd like to do something like the following in express:

The route middleware
```
function (req, res, next) {
   res.on ('end', function () {
      // log the response code and handle db
      if (res.statusCode < 400) { db.commit() } else { db.rollback() }
   });
   next();
}
```
The route:
```
app.post ("/something", function (req, res) { 
    db.doSomething (function () {
       if (some problem) {
          res.send (500);
       } else {
          res.send (200);
       }
    });
 }
```
When I try this, the 'end' event handler never gets called. The same for res.on('close'), which I read about in another post. Do such events get fired?

The only other way I can think of doing this is wrapping res.end or res.send with my own version in a custom middleware. This is not ideal, because res.end and res.send don't take callbacks, so I can't just wrap them, call the original and then do my thing based on the response code that got set when they call me back (because they won't call me back).

Is there a simple way to do this?
Jake about 12 years

Thanks. From your answer, it sounds like the way I'd like to do things is simply impossible without replacing res.send with my own version that does the cleanup then calls the real res.send.
Florian Margaine about 12 years

Yes, res.send() is the last thing your app will do when processing the request, so if you want any cleanup you have to do it before :)
agent-j over 11 years

+1 I couldn't find the documentation, but the current source code (as of today) still raises the event. Thanks!
Hadesara over 11 years

Cant make it work with finish or end or close. any suggestions?
Hadesara over 11 years

res.on ('header',func) FTW!
Hadesara over 11 years

res.on ('header',func) FTW!
S.D. about 10 years

Is this the documented 'finish' event ? : nodejs.org/api/http.html#http_event_finish
Fluffy about 9 years

Express.js response extends built-in http response, and finish event is not express-specific, therefore it's not in express docs
Matthew Herbst almost 8 years

Is there any way to get access to the final state of res within the callback? For example, could the callback look into res and pull out data that was sent to the client?
weexpectedTHIS almost 8 years

Yes @MatthewHerbst. Since this event listener is being handled inside the app.use call, just use the same "res" object from there.
Lee Jenkins over 3 years

The answer is incorrect. See answers below for using res.on('finish',...) to attach an event handler to fire when the response is sent.