使用Express合并,缩小和提供JavaScript文件,但响应不是gzip压缩

我正在使用Express来构建一个Web应用程序.我想合并,缩小和提供.js文件.我写了一个中间件,这是我的代码:

var fs = require('fs'),
    path = require('path'),
    uglify = require('uglify-js'),
    cache = '',
    scriptsDir = path.join(__dirname, '../scripts'),
    files = fs.readdirSync(scriptsDir);

// Sync read is not a problem since the following code is executed on startup
files.forEach(function(fname) {
    if (/^.*\.js$/.test(fname)) {
        cache += fs.readFileSync(path.join(scriptsDir, fname), 'utf8').toString();
    }
});
cache = uglify.minify(cache, { fromString: true }).code;

module.exports = function(req, res, next) {
    if (req.url === '/js/all.js')
        res.end(cache);
    else
        next();
};

中间件以这种方式使用:

app.use(compress());
[...]
app.use(app.router);
app.use(jsMerger); // Here is my middleware
app.use(express.static(path.join(__dirname, '../public')));

问题是响应不是gzip压缩.此外,响应中有"无标题"(我的意思是,没有缓存标头,没有标签; static中间件提供的其他资源都有这些标头).这是回应:

X-Powered-By: Express
Transfer-Encoding: chunked
Date: Wed, 12 Mar 2014 14:04:19 GMT
Connection: keep-alive

我错过了什么吗？如何压缩响应？