Nodejs: Connect and “writeHead” in first connect-callback

问题

I receive always the error "Error: Can't set headers after they are sent." when calling "res.writeHead(...)" in the very first callback of connect:

var http = require('http');
var connect = require('connect');

var simpleApp = connect();

simpleApp
    .use(function(req, res, next){

        res.writeHead(200, { 'content-type': 'text/html' });
        res.write('response powered by SIMPLE connect-object as middelware');

        console.log('Pre');
        next();
        console.log('Post');
    })
    .use(function(req, res, next){
        console.log('I am the header guy!');
        next();
    })
    .use('/admin', function(req, res, next){
        console.log('someone entered the admin area....');
        next();
    })
    .use(function(req, res){
        console.log('reached the tail of the "chain of responsibility!!!');
        res.end();
    });

http.createServer(simpleApp).listen(process.env.PORT || 3000);

console.log('Running on port "' + (process.env.PORT || 3000) + '"');

// just a save-guard to stop the process after some time...
setTimeout(function(){
    process.exit(0);
}, 20000);

And this is the error-message:

Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (http.js:691:11)
    at ServerResponse.res.setHeader (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\patch.js:63:22)
    at next (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\proto.js:156:13)
    at Object.handle (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\connectSampleApp.js:13:3)
    at next (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\proto.js:193:15)
    at Function.app.handle (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\proto.js:201:3)
    at Server.app (C:\Users\drt\SkyDrive\Programmierung\nodejs\silkveil\node_modules\connect\lib\connect.js:65:37)
    at Server.EventEmitter.emit (events.js:98:17)
    at HTTPParser.parser.onIncoming (http.js:2108:12)

When I move the "writeHead"-code into the very last callback everything is fine:

.use(function(req, res){
        console.log('reached the tail of the "chain of responsibility!!!');
        res.writeHead(200, { 'content-type': 'text/html' });
        res.write('response powered by SIMPLE connect-object as middelware');
        res.end();
    });

So my question is: Is it only permitted to use writeHead /write in the very last callback when using connect?

回答1:

check the other question. res.writeHead Basically after res.writeHead is called, the header could not be modified any more, while next method would try to modify the header which will cause the exception.

So you can modify the header in the first connect callback, but you are not allowed to write the body(res.write). Following code should work correctly. In short, you can modify the header but not to flush them.

var http = require('http');
var connect = require('connect');

var simpleApp = connect();

simpleApp
    .use(function(req, res, next){
        res.statusCode = 200;
        res.setHeader( 'content-type', 'text/html' );

        console.log('Pre');
        next();
        console.log('Post');
    })
    .use(function(req, res, next){
        console.log('I am the header guy!');
        next();
    })
    .use('/admin', function(req, res, next){
        console.log('someone entered the admin area....');
        next();
    })
    .use(function(req, res){
        res.write('response powered by SIMPLE connect-object as middelware');
        console.log('reached the tail of the "chain of responsibility!!!');
        res.end();
    });

http.createServer(simpleApp).listen(process.env.PORT || 3000);

console.log('Running on port "' + (process.env.PORT || 3000) + '"');

// just a save-guard to stop the process after some time...
setTimeout(function(){
    process.exit(0);
}, 20000);

回答2:

You forgot this line in your first part of code:

res.end();

If that isn't working there is comment from TJ Holowaychuk here: https://github.com/senchalabs/connect/issues/683#issuecomment-10018892

来源：https://stackoverflow.com/questions/19980426/nodejs-connect-and-writehead-in-first-connect-callback

标签

node.js

connect