Middleware functions are functions that have access to the request object (req),
the response object (res), and the next middleware function in the application’s
request-response cycle. Middleware functions can execute any code, make changes
to res and req objects, end response cycle and call next middleware.
Very common example of middleware is cors module. To add CORS support, simply
install it, require it and put this line:
app.use(cors());
// Or another common middleware
app.use(express.json());before any routers or routing functions.