What is middleware?

Middleware is any number of functions that are invoked by the Express.js routing layer before your final request handler is, and thus sits in the middle between a raw request and the final intended route. We often refer to these functions as the middleware stack since they are always invoked in the order they are added.

var app = express();

app.get('/', function(req, res) {
  res.send('Hello World!');
});

app.get('/info', function(req, res) {
  res.send('data');
});

Request-response lifecycle through a middleware is as follows:

• The first middleware function (A) in the pipeline will be invoked to process the request

• Each middleware function may end the request by sending response to client or invoke the next middleware function (B) by calling next() or hand over the request to an error-handling middleware by calling next(err) with an error argument

• Each middleware function receives input as the result of previous middleware function

• If the request reaches the last middleware in the pipeline, we can assume a 404 error

app.use((req, res) => {
    res.writeHead(404, { 'Content-Type': 'text/html' });
    res.end("Cannot " + req.method.toUpperCase() + " " + req.url);
});

This pattern has some benefits:

• Avoid coupling the sender of a request to the receiver by giving more than one object a chance to handle the request. Both the receiver and the sender have no explicit knowledge of each other.
• Flexibility in distributing responsibilities among objects. We add or change responsibilities for handling a request by adding to or changing the chain at run-time.