How do I play nice with Node.js Express compress middleware?

Question

I want to use some middleware to trim all whitespace between HTML tags and collapse all other whitespace to a single space. This is to aid with CSS because white-space-collapse: discard; isn't widely available (if at all?) and I'm not a fan of other workarounds . I'm fine with a naive approach for now -- but I do want it to play nice with the express.compress middleware.

This is what I have:

module.exports = function trimmer() {
    function getSize(chunk) {
        return Buffer.isBuffer(chunk)
        ? chunk.length
        : Buffer.byteLength(chunk);
    }

    return function trimmer(req, res, next) {
        var end = res.end
            , write = res.write
            , isHtml
        ;

        res.on('header', function() {
            //res.removeHeader('Content-Length'); // another thing I've tried; don't entirely understand it though
        });

        res.write = function(chunk, encoding) {
            var type = res.getHeader('Content-Type') || '';
            isHtml = type.indexOf('text/html') >= 0;
            if (!isHtml) {
                write.apply(res, arguments);
                return;
            }

            var html = chunk
                .toString(encoding)
                .replace(/>\s+</g, '><')
                .replace(/\s{2,}/g, ' ')
            ;

            var buffer = new Buffer(html, encoding);

            try {
                res.setHeader('Content-Length', getSize(buffer));
            } catch (ex) {}
            return write.call(res, buffer, encoding);
        };

        next();
    };
};

This works perfectly fine like so:

app.configure(function() {
    app.use(trimmer());
    // app.use(express.compress()); // till I uncomment this line... then it breaks
    app.use(express.favicon());
    app.use('/images',  express.static(images));
    app.use('/scripts', express.static(scripts));
    app.use(less({ src: pub, dest: tmp }));
    app.use(express.static(tmp));
    app.use(express.static(views));
});

Uncommenting the noted line above causes exceptions related to not being able to modify headers that've already been sent. That's fair, I understand that. I looked at compress 's source code and it was a little above my head. What do I have to do/monkeypatch to not step on compress 's toes (and vice-versa)?

Answer 1

Have you tried putting app.use(trimmer()); below app.use(express.compress()); ? The way it's currently written, trimmer will be called after the response is compressed; switching the order ensures that (1) you're not trying to trim compressed data, and (2) the results of your trimming will be properly compressed.