how to install mongoose

$ npm install mongoose

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/twitter', {
    useNewUrlParser: true,
    useUnifiedTopology: true
});

const db = mongoose.connection;

db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function () {
    console.log("Database Connected !!!");
});

module.exports = db;

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/my_database', {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    useFindAndModify: false,
    useCreateIndex: true
  })
  .then(()=> console.log('connect'))
  .catch((error) => console.error(error));