Hijack response before sent to client - node.js

Question

I have follow codes to be used as middlewares
module.exports=function(callback) {
callbacks.push(callback);
return function(req,res,next) {
if (!res.hijacked) {
res.hijacked=true;
} else {
return next();
}
var send=res.send;
res.send=function(data) {
var body=data instanceof Buffer ? data.toString() : data;
var requests=[];
requests.push(function(next) {
callbacks[0](req,res)(body,doneWrapper(body,next));
});
for (var i=1;i<callbacks.length;i++) {
var hijackCallback=callbacks[i];
requests.push(function(result,next) {
hijackCallback(req,res)(result,doneWrapper(result,next));
});
}
var that=this;
async.waterfall(requests,function(err,result) {
send.call(that,result);
requests=null;
body=null;
that=null;
});
};
next();
};
};
An example of usage is as following:
module.exports=function() {
return hijack(function() {
return function(result,done) {
var json={};
try {
json=JSON.parse(result);
} catch(e) {
return done();
}
if (!_.isArray(json)) {
return done();
}
var sorted=_(json).sortBy(function(item) {
if (_.isObject(item.information)) {
return item.information.rangeIndex1 || 999;
} else {
return 1001;
}
});
done(sorted);
}
});
};
It worked fine initially as middlewares in routes.
However,When i try to make it as app.use(hijackMiddleware()). Something went wrong, I got this Can't set headers after they are sent error.
There is no problem when used as middlewares in routes,though.

Answer 1

Have you consider using express-interceptor? Is really easy to use:
var express = require('express');
var cheerio = require('cheerio');
var interceptor = require('express-interceptor');
var app = express();
var finalParagraphInterceptor = interceptor(function(req, res){
return {
// Only HTML responses will be intercepted
isInterceptable: function(){
return /text\/html/.test(res.get('Content-Type'));
},
// Appends a paragraph at the end of the response body
intercept: function(body, send) {
var $document = cheerio.load(body);
$document('body').append('<p>From interceptor!</p>');
send($document.html());
}
};
})
// Add the interceptor middleware
app.use(finalParagraphInterceptor);
app.use(express.static(__dirname + '/public/'));
app.listen(3000);