koa-lite/examples/conditional-middleware-middleware.js

var koa = require('..');
var app = koa();

// logger

function *logger(next){
  var start = new Date;
  yield next;
  var ms = new Date - start;
  console.log('%s %s - %s', this.method, this.url, ms);
}

// passing any middleware to this middleware
// will make it conditional, and will not be used
// when an asset is requested. This is a modular
// approach to conditiona-middleware.js

function ignoreAssets(mw) {
  return function *(next){
    if (/(\.js|\.css|\.ico)$/.test(this.path)) {
      yield next;
    } else {
      yield mw(next);
    }
  }
}

app.use(ignoreAssets(logger));

// sometimes it's useful to apply some
// ad-hoc logic to enable middleware, for
// example ignoring a logger on asset requests:

app.use(function *(){
  this.body = 'Hello World';
});

app.listen(3000);