Just some code example

db.js

/**
 * @param dbType {('mysql'|'mssql'|'postgresql')} Type of the db
 * @constructor
 */
function DB(dbType) {
    if (['mysql', 'mssql', 'postgresql'].indexOf(dbType) === -1) {
        throw new Error('Unsupported database type');
    }

    /**
     * @type {('mysql'|'mssql'|'postgresql')} Type of the db
     * @private
     */
    this._dbType = dbType;

    this.connect = function () {
        console.log(`Connecting to ${this._dbType}`);

        return new Promise(resolve => {
            setTimeout(() => {
                console.log(`Connected to ${this._dbType}`);

                this.connectedAt = new Date();
                resolve();
            }, 500);
        });
    }
}

export default DB;

index.js

import DB from './db';

const db = new DB("mssql");
db.connect();
console.log(db.connectedAt);