• Middleware: Whenever server gets request, it needs to see if it is valid.

: To check something, we need to put codes about authorization information whenever we meet a router.

: We can define what we need to control before something et to router. And it is MIDDLEWARE.

app.use((req, res, next) => {  // middleware before going to router.
    console.log(req);  // see all request
    next();  // finish middleware and go to next middleware.
});

• express.json(), express.urlencoded()

: When client send requests, server can send more than simple urls like 'body' in 'POST' method.

: express.json() makes it easier to manufacture the data

app.use(express.urlencoded({extended: false}));
app.use(express.json())

• express.static()

: This makes client can see static files.

app.use(express.static('public'));  // to offer static files