Express.js render Stylus .styl to .css every request; skips output file to local disk

routes.js

// render .styl to .css every request; skips output file to local disk
app.use((req, res, next) => {
    const stylus = require('stylus');
    const join = require('path').join;
    const src = join(__dirname, '/../public');
    const url = require('url');
    const fs = require('fs');
    if ('GET' !== req.method && 'HEAD' !== req.method) return next();
    const path = url.parse(req.url).pathname;
    if (!/\.css$/.test(path)) return next();
    const stylusPath = join(src, path.replace('.css', '.styl'));
    const error = err => next('ENOENT' === err.code ? null : err);
    fs.readFile(stylusPath, 'utf8', (err, str) => {
        if (err) return error(err);
        stylus(str)
            .set('filename', stylusPath)
            .render((err, css) => {
                if (err) return error(err);
                res.type('css').end(css);
            });
    });
});