The order in which you use middleware in Express matters: middleware declared earlier will get called first, and if it can handle a request, any middleware declared later will not get called.
If express.static is handling the request, you need to move your middleware up:
// need cookieParser middleware before we can do anything with cookies
app.use(express.cookieParser());
// set a cookie
app.use(function (req, res, next) {
// check if client sent cookie
var cookie = req.cookies.cookieName;
if (cookie === undefined) {
// no: set a new cookie
var randomNumber=Math.random().toString();
randomNumber=randomNumber.substring(2,randomNumber.length);
res.cookie('cookieName',randomNumber, { maxAge: 900000, httpOnly: true });
console.log('cookie created successfully');
} else {
// yes, cookie was already present
console.log('cookie exists', cookie);
}
next(); // <-- important!
});
// let static middleware do its job
app.use(express.static(__dirname + '/public'));
Also, middleware needs to either end a request (by sending back a response), or pass the request to the next middleware. In this case, I've done the latter by calling next() when the cookie has been set.
Update
As of now the cookie parser is a seperate npm package, so instead of using
app.use(express.cookieParser());
you need to install it separately using npm i cookie-parser and then use it as:
const cookieParser = require('cookie-parser');
app.use(cookieParser());