Middleware are functions that are run after a request is received but before a response is returned.
More explicitly
Middleware functions have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.
Middleware functions can perform the following tasks:
- Execute any code.
- Make changes to the request and the response objects.
- End the request-response cycle.
- Call the next middleware function in the stack.
If the current middleware function does not end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.
Don't forget to npm install your middleware before running it in your code!
 body-parser: Extracts the body of an incoming request stream and puts it into req.body. When parsing JSON bodies, it puts the key-value pairs into req.body. To use it:
var bodyParser = require('body-parser');
app.use(bodyParser.json());
 compression: Attempts to compress response bodies for any requests that pass through it. How heavily to compress is configurable (more compression => slower).
var compression = require('compression');
app.use(compression());
 cookie-parser: Allows you to sign cookies, parse cookies, and check if cookies are authentic (signed).
var cookieParser = require('cookie-parser');
app.use(cookieParser());
 helmet: Helmet helps you secure your  apps by setting various HTTP headers.
It's best to use Helmet early in your middleware stack so that its headers are sure to be set. ie:
const helmet = require('helmet');
app.use(helmet());
We bind application-level middleware to an instance of the app object by using the app.use() and app.METHOD() functions, where METHOD is the HTTP method of the request that the middleware function handles (such as GET, PUT, or POST) in lowercase.
This example shows a middleware function with which will be applied to every route (as none is supplied). The function is executed every time the app receives a request.
var app = express();
app.use(function (req, res, next) {
console.log('Time:', Date.now());
next();
});
If a route is supplied as the first argument to app.use the middleware will only be applied to the specified route(s). The route can be supplied as a string, a path pattern or a regex, or an array containing any combination of the above.
- Middleware are always invoked in the order in which they are added.
- Middleware can add to and modify properties on the request and response objects - but all middleware receive the same instance of these objects, which can cause conflicts if multiple middleware change the same property in different ways.