piler.js

var stylus = require('stylus')
  , nib = require('nib')
  , uglify = require('uglifyjs')

function processStylus(input, cb) {

  fs.readFile(input.src, function (err, data) {

    if (err) return cb(err)

    stylus
      .use(nib())
      .set('compress', true)
      .set('more options', { foo: 1 })
      .define('my function', function () { /* ... */ })
      .compile(data, function (err, css) {
        if (err) return cb(err)
        fs.writeFile(input.dest, function (err) {
          if (err) return cb(err)
          cb(null, 'yay')
      })
  })

}

function processJS(input, cb) {

  // The string of concated js
  var out = ''

  function write() {
    fs.writeFile(input.dest, out, function (err) {
      if (err) return cb(err)
      cb(null, 'yay')
    })
  }

  // Determine whether to bundle or provide loader
  if (someFlag) {

    // This makes the assumption that the paths in srcs are the
    // the same as the public available web path. (but they are not)
    out += buildLoader(input.srcs)

  } else {

    var done = 0

    input.srcs.forEach(function (src) {
      fs.readFile(src, function (err, data) {
        out += uglify(data)
        if (++done === input.srcs.length) write()
      })
    })

  }

}

module.exports =
  [ { items:
      [ { src: 'public/css/screen.styl'
        , dest: 'public/css/screen.css'
        }
      ]
    , process: processStylus
    }
  , { items:
      [ { dest: 'public/js/app.js'
        , srcs: [ 'public/js/a.js', 'public/js/b.js' ]
        }
      ]
    , process: processJS
    }