index.js

require('dotenv').config();
const express = require('express');
const cors = require('cors');
const app = express();

let bodyParser = require('body-parser');
app.use(bodyParser.urlencoded({ extended: false }));

const url = require('url');

let id = 2;
let urlMap = {
  "https://www.baidu.com": 0,
  "https://www.freecodecamp.org": 1
};

const dns = require('dns');

// Basic Configuration
const port = process.env.PORT || 3000;

app.use(cors());

app.use('/public', express.static(`${process.cwd()}/public`));

app.get('/', function(req, res) {
  res.sendFile(process.cwd() + '/views/index.html');
});

// Your first API endpoint
app.post('/api/shorturl', (req, res) => {
  const originalUrl = req.body.url;
  const urlParse = url.parse(originalUrl);
  if (!urlParse.hostname) res.json({"error": "invalid url"});
  else {
    dns.lookup(urlParse.hostname, (err, address) => {
      if (err) res.json({"error": "invalid url"});
      else {
        if (!urlMap[originalUrl]) {
          urlMap[originalUrl] = id;
          id++;
        }
        res.json(
          {
            original_url: originalUrl,
            short_url: urlMap[originalUrl]
          }
        );
      }
    })
  }
})

app.get('/api/shorturl/:shortUrl', function(req, res) {
  const originalUrl = Object.keys(urlMap).find(key => urlMap[key]==req.params.shortUrl);
  if (originalUrl) res.redirect(originalUrl);
  else res.json({error: "No short URL found for the given input"});
});

app.listen(port, function() {
  console.log(`Listening on port ${port}`);
});