Express.js: Parsing Predomain from URL works with localhost but not with 127.0.0.1

I’ve implemented a simple Express.js application that extracts the predomain from incoming requests’ hostnames. The code seems to be functioning correctly when I use localhost:3000, but it fails when I try 127.0.0.1:3000.

Here’s a snippet of the code I’m using:

const express = require('express');
const app = express();

app.use((req, res, next) => {
  const parts = req.hostname.split('.');
  req.predomain = parts[0];
  next();
});

app.get("https://stackoverflow.com/", (req, res) => {
  res.json({ predomain: req.predomain });
});

const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server is running on port ${PORT}`);
});

When I access http://harsh.localhost:3000, the API correctly returns the predomain as expected (harsh). However, when I access http://harsh.127.0.0.1:3000 the postman or chrome cannot find the address itself. I’m curious as to why this might be happening and how I can modify my code to correctly handle the extraction of the predomain even when using 127.0.0.1 or something else.

The issue here is conceptual, not with your code – unlike a domain that may have subdomains, an IP (v4) address is always four integers delimited by dots. You simply can’t add a subdomain to an IP address.