Not able to access node.js express endpoints in browser, but postman works just fine

Question

I am working on an express app locally. I have a route in my app that looks like

rootRouter.get('/', async (_: Request, res: Response) => {
  res.status(OK).send('Hello!');
});

Command line run

node -r module-alias/register ./dist --env=development

Browser behavior:

I cannot see the route through my browser localhost:6000. I see "This site can’t be reached". It's like that for all URLs. I don't see anything appear in my server logs.

Postman behavior:

When I hit that same route localhost:6000 with a get request in postman, I see the text "Hello!". Logs - GET / 200 0.354 ms - 25

Heroku run

heroku local web -p=5000 (calls the same node -r module-alias/register ./dist --env=development as above, but uses port 5000 instead)

Browser behavior:

I CAN see the route through my browser localhost:5000. I see "Hello!". I see the request in my logs

8:44:45 PM web.1 |  GET / 200 0.517 ms - 25
8:44:45 PM web.1 |  GET /favicon.ico 200 1.928 ms - 12014

Postman behavior:

Also works, I see "Hello!", logs - 9:01:29 PM web.1 | GET / 200 0.193 ms - 25

Rough server code:

const app = express();

app.use(express.json());
app.use(express.urlencoded({extended: true}));
app.use(cookieParser());
app.use(morgan('dev'));
app.use('/', rootRouter); //contains my route that I posted above

// error handling
app.use((err: Error, _: Request, res: Response, __: NextFunction) => {
  const message = err.message;
  return res.status(500).json({ error: message });
});

const viewsDir = path.join(__dirname, 'views');
app.set('views', viewsDir);

const staticDir = path.join(__dirname, 'public');
app.use(express.static(staticDir));

Answer 1

The Problem

I realized what the problem was. Specifically, port 6000 is forbidden by many browsers. https://chromium.googlesource.com/chromium/src.git/+/refs/heads/master/net/base/port_util.cc

It was firefox that helped me figure it out - since their error message is much more specific than chromes.

Firefox's error message:

"This address is restricted This address uses a network port which is normally used for purposes other than Web browsing. Firefox has canceled the request for your protection."