old_contents: var PouchDB = require('pouchdb');
  var KakuDB = new PouchDB('kaku');
  
  // Note:
  // Because we add something new in the prototype chain and this is not safe,
  // we added some prefix to make it unique
  PouchDB.prototype.resetDatabase = function() {
    return this.destroy().catch((error) => {
      console.log('Something goes wrong when dropping database');
      console.log(error);
    });
  };
  
  module.exports = KakuDB;
  
new_contents: const PouchDB = require('pouchdb');
  const KakuDB = new PouchDB('kaku');
  
  // Note:
  // Because we add something new in the prototype chain and this is not safe,
  // we added some prefix to make it unique
  PouchDB.prototype.resetDatabase = function() {
    return this.destroy().catch((error) => {
      console.log('Something goes wrong when dropping database');
      console.log(error);
    });
  };
  
  module.exports = KakuDB;
  
subject: rewrite src/modules/Database.js with ES6 syntax