Node.js – In Express, what does app.router do exactly

expressnode.js

When I create a sample Express application using the express binary, the bootstrap code has these lines:

...

var app = express();
...
app.use(app.router);

I didn't find much about app.router. I thought that this is the middleware that handles the routing (app.get(), app.post() etc.) rules, but these rules also get executed when I remove the app.use(app.router); line.

So what is the exact purpuse of this middleware?

Best Answer

In Express 3.x, app.router is an enhanced version of the connect middleware router. As hector said, this is where Express handles the request handlers registered with app.get, app.post, etc.

If you do not call app.use(app.router) explicitly then express will call it implicitly the first time you use app.get(...), app.post(...), etc. However, you may want to .use it explicitly, because then you choose the order of all your middleware.

app.use(express.favicon());
app.use(express.bodyParser());
app.use(express.methodOverride());
// app.get, app.post, etc called before static folder
app.use(app.router); 
app.use(express.static(path.join(__dirname, 'public')));

See how the router is retrieved in the Express 3 source here.

Note that Express 4 doesn't need app.router.

Related Solutions

Node.js – Node.js’ Connect, Express and “middleware”

[Update: As of its 4.0 release, Express no longer uses Connect. However, Express is still compatible with middleware written for Connect. My original answer is below.]

I'm glad you asked about this, because it's definitely a common point of confusion for folks looking at Node.js. Here's my best shot at explaining it:

Node.js itself offers an http module, whose createServer method returns an object that you can use to respond to HTTP requests. That object inherits the http.Server prototype.
Connect also offers a createServer method, which returns an object that inherits an extended version of http.Server. Connect's extensions are mainly there to make it easy to plug in middleware. That's why Connect describes itself as a "middleware framework," and is often analogized to Ruby's Rack.
Express does to Connect what Connect does to the http module: It offers a createServer method that extends Connect's Server prototype. So all of the functionality of Connect is there, plus view rendering and a handy DSL for describing routes. Ruby's Sinatra is a good analogy.
Then there are other frameworks that go even further and extend Express! Zappa, for instance, which integrates support for CoffeeScript, server-side jQuery, and testing.

Here's a concrete example of what's meant by "middleware": Out of the box, none of the above serves static files for you. But just throw in connect.static (a middleware that comes with Connect), configured to point to a directory, and your server will provide access to the files in that directory. Note that Express provides Connect's middlewares also; express.static is the same as connect.static. (Both were known as staticProvider until recently.)

My impression is that most "real" Node.js apps are being developed with Express these days; the features it adds are extremely useful, and all of the lower-level functionality is still there if you want it.

Javascript – the purpose of Node.js module.exports and how do you use it

module.exports is the object that's actually returned as the result of a require call.

The exports variable is initially set to that same object (i.e. it's a shorthand "alias"), so in the module code you would usually write something like this:

let myFunc1 = function() { ... };
let myFunc2 = function() { ... };
exports.myFunc1 = myFunc1;
exports.myFunc2 = myFunc2;

to export (or "expose") the internally scoped functions myFunc1 and myFunc2.

And in the calling code you would use:

const m = require('./mymodule');
m.myFunc1();

where the last line shows how the result of require is (usually) just a plain object whose properties may be accessed.

NB: if you overwrite exports then it will no longer refer to module.exports. So if you wish to assign a new object (or a function reference) to exports then you should also assign that new object to module.exports

It's worth noting that the name added to the exports object does not have to be the same as the module's internally scoped name for the value that you're adding, so you could have:

let myVeryLongInternalName = function() { ... };
exports.shortName = myVeryLongInternalName;
// add other objects, functions, as required

followed by:

const m = require('./mymodule');
m.shortName(); // invokes module.myVeryLongInternalName

Best Answer

Related Solutions

Node.js – Node.js’ Connect, Express and “middleware”

Javascript – the purpose of Node.js module.exports and how do you use it

Related Topic