to understand the middlewares of express better

simplify-express-middleware-model.js

var proto = {};

function express() {
  var app = function(ctx, cb) {
    app.handle(ctx, cb);
  }
  app.__proto__ = proto;
  app.stack = [];
  return app;
}

proto.use = function (fn) {
  this.stack.push(fn);
}

proto.handle = function (ctx, out) {
  var self = this;
  var index = 0;
  var done = out || function() {};
  function next(error) {
    // error handler first

    // get fn in stack serially
    var fn = self.stack[index++];

    // if no function in stack, done
    if (!fn) {
      setImmediate(done);
      return;
    }

    /**
     * a middleware function is defined as follow:
     * function(ctx, next) {
     *    do something..
     *    next();
     *    do something else..
     * }
     */
    fn(ctx, next);
  }

  next();
}

// -------------------- test --------------------
var app = express();
app.use(function(ctx, next) {
  ctx.a = 1;
  next();
  console.log("test");
});

app.use(function(ctx, next) {
  ctx.b = 2;
  console.log(ctx);
  next();
});

// in express, should be app(req, res), and `req` `res` is given by http module
app({}, function(){
  console.log('every thing is ok');
});

// the out put is
// { a: 1, b: 2 }
// test
// every thing is ok