How to have a NodeJS/connect middleware execute after responde.end() has been invoked?

Question

I would like to achieve something like this:

var c = require('connect');
var app = c();

app.use("/api", function(req, res, next){
    console.log("request filter 1");
    next();
});

app.use("/api", function(req, res, next){
    console.log("request filter 2");
    next();
});

app.use("/api", function(req, res, next){
   console.log("request handler");
    res.end("hello");
    next();
});

app.use("/api", function(req, res, next){
    console.log("response post processor");
    next();
});
app.listen(3000);

When I curl for the address, I get an exception to the console complaining about headers cannot be bothered after being sent which is fair enough. Only that I do not touch the response object.

/usr/bin/node app2.js
request filter 1
request filter 2
request handler
Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (http.js:644:11)
    at ServerResponse.res.setHeader (/home/zpace/node_modules/connect/lib/patch.js:59:22)
    at next (/home/zpace/node_modules/connect/lib/proto.js:153:13)
    at Object.handle (/home/zpace/WebstormProjects/untitled1/app2.js:25:5)
    at next (/home/zpace/node_modules/connect/lib/proto.js:190:15)
    at Object.handle (/home/zpace/WebstormProjects/untitled1/app2.js:19:5)
    at next (/home/zpace/node_modules/connect/lib/proto.js:190:15)
    at Object.handle (/home/zpace/WebstormProjects/untitled1/app2.js:14:5)
    at next (/home/zpace/node_modules/connect/lib/proto.js:190:15)
    at Function.app.handle (/home/zpace/node_modules/connect/lib/proto.js:198:3)

Debugging the NodeJS/Connect layer I got into a part that somehow implies that if headers are already sent then executing a route handler must initialize response headers.

The question is if the above mentioned behavior is intentional (ie that the execution of any code after a route handler has finished sending a response is something utterly unimaginable or this is simply a bug in connect?

Answer 1

Not sure whether you have found your solution.

If you want to design a post-processor for the request cycle, you can use a middleware that listens to the "finish" event on the response object. Like this:

app.use(function(req, res, next){
  res.on('finish', function(){
    console.log("Finished " + res.headersSent); // for example
    console.log("Finished " + res.statusCode);  // for example
    // Do whatever you want
  });
  next();
});

The function attached to the "finish" event will be executed after the response is written out (which means the NodeJS has handed off the response header and body to the OS for network transmission).

I guess this must be what you want.

Answer 2

I think this is a bad planning problem. You should solve this in a better way. I dont know why you have a request handler and a request post processor separated, but lets find out what we can do.

So yes, after response has ended you cant read the headers again.

So dont finish the response until the post processor is invoked.

var isEnd;

app.use("/*", function(req, res, next){
  isEnd = false;
})

app.use("/api", function(req, res, next){
   console.log("request handler");
    res.write("hello");
    isEnd = true;
    next();
});

app.use("/api", function(req, res, next){
    console.log("response post processor");
    if(isEnd) {
        res.end();
    }
    else next();
});

This is a kind of solution, but this may not be the best for your problem.

In my opinion it is really bad that you call next() after the response has been finished. If you need a post processor, why you do that in a request filterer (or what is this). Call a function but not next()

Maybe this:

app.use("/api", function(req, res, next){
   console.log("request handler");
    res.end("hello");
    setTimeout(function(){(postProcessor(req)},0);
});

function postProcessor(req) {
//doing post process stuff.
//response not needed because already ended.
}

Or this:

app.use("/api", function(req, res, next){
   console.log("request handler");
    res.writed("hello");
    setTimeout(function(){(postProcessor(req)},0);
    // u cant res.end here because setTimeout. 
    //If you dont use setTimeout you can use res.end here, but not bot function.
});

function postProcessor(req, res) {
//doing post process stuff.
res.end();
}

The next() is not for that usage, what you uses.

I hope my answer helps you, but i know it not covers everything, but your answer is not really concrete too.