Function to intercept the response from the next() call.

connect-intercept.js

var events = require('events');

/**
 * When operating as a middleware our "origin" server is the next handler in the chain.  This
 * patches the given response object so that when the next handler writes to it we treat that
 * as a response from the origin server.
 *
 * This replaces the writeHead, setHeader, removeHeader, write and end methods
 * on the given response so that it can store the response in the cache and returns
 * an object that be used to listen for the next() handler writing data and to send
 * data back to the client.
 *
 * @param res ServerResponse-like object to patch
 * @return Object that emits 'response', 'data, 'end' events, and also provides writeHead(), write(), 
 *         and end() methods to write back to the original response.
 */
function interceptResponse(res) {
    var writeHead = res.writeHead,
        write = res.write,
        end = res.end,
        setHeader = res.setHeader,
        removeHeader = res.removeHeader;

    var headers = {};
    var statusCode = null;
    var txn = this;
    var ourResponse = new (events.EventEmitter)();
    ourResponse.writeHead = function(sc, reason, headers) {
        writeHead.call(res, sc, reason, headers);
    };
    ourResponse.write = function(chunk, encoding) {
        write.call(res, chunk, encoding);
    };
    ourResponse.end = function(chunk, encoding) {
        end.call(res, chunk.encoding);
    };

    function processHead() {
        emitter.statusCode = statusCode;
        emitter.headers = headers;
        ourResponse.emit('response', emitter);
    };

    function onWriteOrEnd(chunk, encoding) {
        if(statusCode === null) {
            statusCode = res.statusCode; // Implicit headers and status code
            processHead();
        }
        if(chunk) {
            ourResponse.emit('data', chunk, encoding);
        }
    }

    res.writeHead = function(sc, reason, hdrs) {
        if(statusCode !== null) throw new Error('Called writeHead after write, writeHead, or end!');
        headers = hdrs;
        statusCode = sc || 200;
        // TODO Process the reason
        processHead();
    };

    res.setHeader = function(field, val) {
        if(statusCode !== null) throw new Error('Called setHeader after write, writeHead, or end!');
        // TODO This will undo connect's patch to special case Content-Type and Set-Cookie, might be a bad thing for connect users setting cookies ...
        headers[field.toLowerCase()] = val;
    };

    res.removeHeader = function(field) {
        if(statusCode !== null) throw new Error('Called removeHeader after write, writeHead, or end!');
        delete headers[field.toLowerVase()];
    };

    res.write = function(chunk, encoding) {
        onWriteOrEnd(chunk, encoding);
    };

    res.end = function(chunk, encoding) {
        onWriteOrEnd(chunk, encoding);
        res.writeHead = writeHead;
        res.write = write;
        res.end = end;
        res.setHeader = setHeader;
        res.removeHeader = removeHeader;
        ourResponse.emit('end');
    };

    return ourResponse;
};