What is Node.js' Connect, Express and "middleware"?
Despite knowing JavaScript quite well, I'm confused what exactly these three projects in Node.js ecosystem do. Is it something like Rails' Rack? Can someone please explain?
[ 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.
The accepted answer is really old (and now wrong). Here's the information (with source) based on the current version of Connect (3.0) / Express (4.0).
What Node.js comes with
http / https createServer which simply takes a callback(req,res) eg
var server = http.createServer(function (request, response) {
// respond
response.write('hello client!');
response.end();
});
server.listen(3000);
What connect adds
Middleware is basically any software that sits between your application code and some low level API. Connect extends the built-in HTTP server functionality and adds a plugin framework. The plugins act as middleware and hence connect is a middleware framework
The way it does that is pretty simple (and in fact the code is really short!). As soon as you call var connect = require('connect'); var app = connect(); var connect = require('connect'); var app = connect(); you get a function app that can:
.use (source) to manage plugins (that comes from here because of this simple line of code). Because of 1.) you can do the following :
var app = connect();
// Register with http
http.createServer(app)
.listen(3000);
Combine with 2.) and you get:
var connect = require('connect');
// Create a connect dispatcher
var app = connect()
// register a middleware
.use(function (req, res, next) { next(); });
// Register with http
http.createServer(app)
.listen(3000);
Connect provides a utility function to register itself with http so that you don't need to make the call to http.createServer(app) . Its called listen and the code simply creates a new http server, register's connect as the callback and forwards the arguments to http.listen . From source
app.listen = function(){
var server = http.createServer(this);
return server.listen.apply(server, arguments);
};
So, you can do:
var connect = require('connect');
// Create a connect dispatcher and register with http
var app = connect()
.listen(3000);
console.log('server running on port 3000');
It's still your good old http.createServer with a plugin framework on top.
What ExpressJS adds
ExpressJS and connect are parallel projects. Connect is just a middleware framework, with a nice use function. Express does not depend on Connect (see package.json). However it does the everything that connect does ie:
createServer like connect since it too is just a function that can take a req / res pair (source). listen function to register itself with http In addition to what connect provides (which express duplicates), it has a bunch of more features. eg
The middleware is shared
The use function of ExpressJS and connect is compatible and therefore the middleware is shared. Both are middleware frameworks, express just has more than a simple middleware framework.
Which one should you use?
My opinion: you are informed enough ^based on above^ to make your own choice.
http.createServer if you are creating something like connect / expressjs from scratch. http.createServer Most people should just use ExpressJS.
What's wrong about the accepted answer
These might have been true as some point in time, but wrong now:
that inherits an extended version of http.Server
Wrong. It doesn't extend it and as you have seen ... uses it
Express does to Connect what Connect does to the http module
Express 4.0 doesn't even depend on connect. see the current package.json dependencies section
node.js
Node.js is a javascript motor for the server side.
In addition to all the js capabilities, it includes networking capabilities (like HTTP), and access to the file system.
This is different from client-side js where the networking tasks are monopolized by the browser, and access to the file system is forbidden for security reasons.
node.js as a web server: express
Something that runs in the server, understands HTTP and can access files sounds like a web server. But it isn't one.
To make node.js behave like a web server one has to program it: handle the incoming HTTP requests and provide the appropriate responses.
This is what Express does: it's the implementation of a web server in js.
Thus, implementing a web site is like configuring Express routes, and programming the site's specific features.
Middleware and Connect
Serving pages involves a number of tasks. Many of those tasks are well known and very common, so node's Connect module (one of the many modules available to run under node) implements those tasks.
See the current impressing offering:
Connect is the framework and through it you can pick the (sub)modules you need.
The Contrib Middleware page enumerates a long list of additional middlewares.
Express itself comes with the most common Connect middlewares.
What to do?
Install node.js.
Node comes with npm, the node package manager.
The command npm install -g express will download and install express globally (check the express guide).
Running express foo in a command line (not in node) will create a ready-to-run application named foo. Change to its (newly created) directory and run it with node with the command node <appname> , then open http://localhost:3000 and see. Now you are in.