Error while connecting to databse - nodejs and mongodb

I am a biginner to nodejs and mongodb. I encountered the following error while trying to connect to database:

(node:10124) UnhandledPromiseRejectionWarning: MongoNetworkError: failed to connect to server [127.0.0.1:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017] at Pool. (E:\node js workspace\URL-Shortner\server\node_modules\mongodb-core\lib\topologies\server.js:562:11) at emitOne (events.js:116:13) at Pool.emit (events.js:211:7) at Connection. (E:\node js workspace\URL-Shortner\server\node_modules\mongodb-core\lib\connection\pool.js:316:12) at Object.onceWrapper (events.js:317:30) at emitTwo (events.js:126:13) at Connection.emit (events.js:214:7) at Socket. (E:\node js workspace\URL-Shortner\server\node_modules\mongodb-core\lib\connection\connection.js:245:50) at Object.onceWrapper (events.js:315:30) at emitOne (events.js:116:13) at Socket.emit (events.js:211:7) at emitErrorNT (internal/streams/destroy.js:64:8) at _combinedTickCallback (internal/process/next_tick.js:138:11) at process._tickCallback (internal/process/next_tick.js:180:9) (node:10124) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1) (node:10124) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

i am trying to build a url shortner. This is the code.

index.js-

const express = require("express");
const bodyParser = require("body-parser");
const mongoose = require("mongoose");
const mongoURI = "mongodb://localhost/url-shortner";
const connectOptions={
keepAlive: true,
reconnectTries: Number.MAX_VALUE
};
mongoose.connect(mongoURI,connectOptions,(err,db)=>{
if(err) console.log('Error',err);
console.log('Connected to MongoDB');
});
const app = express();
require("./models/UrlShorten");
app.use(bodyParser.json());
require("./routes/urlshorten")(app);
const PORT = 7000;
//Start server on Port 7000
  app.listen(PORT, () => {
console.log(`Server started on port`, PORT);
});

urlshorten.js-

const mongoose = require("mongoose");
const validUrl = require("valid-url");
const UrlShorten = mongoose.model("UrlShorten");
const shortid = require("shortid");
const errorUrl='http://localhost/error';
module.exports = app => {
app.get("/api/item/:code", async (req, res) => {
const urlCode = req.params.code;
const item = await UrlShorten.findOne({ urlCode: urlCode });
if (item) {
  return res.redirect(item.originalUrl);
 } else {
   return res.redirect(errorUrl);
 }
});
 app.post("/api/item", async (req, res) => {
 const { originalUrl, shortBaseUrl } = req.body;
 if (validUrl.isUri(shortBaseUrl)) {
 } else {
   return res
     .status(401)
     .json(
       "Invalid Base Url"
     );
 }
 const urlCode = shortid.generate();
 const updatedAt = new Date();
 if (validUrl.isUri(originalUrl)) {
   try {
     const item = await UrlShorten.findOne({ originalUrl: originalUrl });
     if (item) {
       res.status(200).json(item);
     } else {
       shortUrl = shortBaseUrl + "/" + urlCode;
      // console.log("Shorturl: "+shortUrl);
        const item = new UrlShorten({
         originalUrl,
         shortUrl,
         urlCode,
         updatedAt
       });
       await item.save();
       res.status(200).json(item);
     }
    } catch (err) {
    res.status(401).json("Invalid User Id");
   }
  } else {
   return res
     .status(401)
     .json(
       "Invalid Original Url"
     );
 }
});
};

This is the schema:

Schema({
originalUrl: String,
urlCode: String,
shortUrl: String,
createdAt: { type: Date, default: Date.now },
updatedAt: { type: Date, default: Date.now }

});

Thank you.