What does middleware and app.use actually mean in Expressjs?

Almost every Express app I see has an app.use statement for middleware but I haven't found a clear, concise explanation of what middleware actually is and what the app.use statement is doing. Even the express docs themselves are a bit vague on this. Can you explain these concepts for me please?

middleware

I'm halfway through separating the concept of middleware in a new project.

Middleware allows you to define a stack of actions that you should flow through. Express servers themselves are a stack of middlewares.

// express
var app = express();
// middleware
var stack = middleware();

Then you can add layers to the middleware stack by calling .use

// express
app.use(express.static(..));
// middleware
stack.use(function(data, next) {
  next();
});

A layer in the middleware stack is a function, which takes n parameters (2 for express, req & res) and a next function.

Middleware expects the layer to do some computation, augment the parameters and then call next.

A stack doesn't do anything unless you handle it. Express will handle the stack every time an incoming HTTP request is caught on the server. With middleware you handle the stack manually.

// express, you need to do nothing
// middleware
stack.handle(someData);

A more complete example :

var middleware = require("../src/middleware.js");

var stack = middleware(function(data, next) {
    data.foo = data.data*2;
    next();
}, function(data, next) {
    setTimeout(function() {
        data.async = true;
        next();
    }, 100)
}, function(data) {
    console.log(data);
});

stack.handle({
    "data": 42
})

In express terms you just define a stack of operations you want express to handle for every incoming HTTP request.

In terms of express (rather than connect) you have global middleware and route specific middleware. This means you can attach a middleware stack to every incoming HTTP requests or only attach it to HTTP requests that interact with a certain route.

Advanced examples of express & middleware :

// middleware 

var stack = middleware(function(req, res, next) {
    users.getAll(function(err, users) {
        if (err) next(err);
        req.users = users;
        next();  
    });
}, function(req, res, next) {
    posts.getAll(function(err, posts) {
        if (err) next(err);
        req.posts = posts;
        next();
    })
}, function(req, res, next) {
    req.posts.forEach(function(post) {
        post.user = req.users[post.userId];
    });

    res.render("blog/posts", {
        "posts": req.posts
    });
});

var app = express.createServer();

app.get("/posts", function(req, res) {
   stack.handle(req, res); 
});

// express

var app = express.createServer();

app.get("/posts", [
    function(req, res, next) {
        users.getAll(function(err, users) {
            if (err) next(err);
            req.users = users;
            next();  
        });
    }, function(req, res, next) {
        posts.getAll(function(err, posts) {
            if (err) next(err);
            req.posts = posts;
            next();
        })
    }, function(req, res, next) {
        req.posts.forEach(function(post) {
            post.user = req.users[post.userId];
        });

        res.render("blog/posts", {
            "posts": req.posts
        });
    }
], function(req, res) {
   stack.handle(req, res); 
});

After simplifying things, a web server can be seen as a function that takes in a request and outputs a response. So if you view a web server as a function, you could organize it into several pieces and separate them into smaller functions so that the composition of them will be the original function.

Middlewares are the smaller functions that you can compose with others and the obvious benefit is that you can reuse them.

What does middleware and app.use actually mean in Expressjs?

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