Question

Is there a way to inject middleware in an Express stack? What I mean is I want to have my app.js setup the main middleware chain, and then call other modules passing the app instance and they may want to insert more middleware (e.g. an authentication module that wants to add passport in at the correct place)

Answer 1

You can certainly pass your app object to other modules and call use there. Of course, middleware functions are executed in the order they are added, so you have to take great care to ensure that you call use in the correct order.

app.js

var app = express();
// ...

app.use(express.logger()); // first middleware function

var someOtherModule = require('./mod.js');
someOtherModule.init(app);

app.use(express.static()); // last middleware function)

mod.js

exports.init = function(app) {
    app.use(function(req, res, next) {

    });
};

---

As far as actually injecting a middleware function in the middle of the stack (after you’ve already called app.use with a set of middleware functions), there’s no documented way to do it. use only adds a function to the end of the stack.

use is actually supplied by Connect in proto.js:

app.use = function(route, fn){

  ...

  this.stack.push({ route: route, handle: fn });

  return this;
};

Technically, you could fiddle with app.stack yourself, but I would not do this. You’d be messing with an undocumented implementation detail, which is liable to change. In other words, it’s possible a future update to either Connect or Express could break your app.