Express application allows to open only one database connection pool object across the application. 

In this example,

1/ mssql library is used to connect MS-SQL server

2/ one time connection pool is generated in the library file "commonDbConnection.js"

3/ above library is referred where ever its required.  In our example, used it in "server.js" & "anotherlibraryfile.js"

config.js

var config = {
 databaseConn: {
 server: "dbserver",
 database: "db",
 user: "uid",
 password: "pwd",
 options: {
 encrypt: false
 }
 },

Config_Key1: 'config_value1',

Config_Key2: 'config_value2',

Config_Obj: {
  config_key3 : 'config_value3',
  config_key4 : 'config_value4',
  config_key5 : 'config_value5',
}

};

module.exports = config ;

commonDbConnection.js

It returns as a promise object "commonConnPool"

var config = require("./config").databaseConn;

const mssql = require("mssql");

const commonConnPool = new mssql.ConnectionPool(config)
 .connect()
 .then(pool => {
 console.log("Connected to MSSQL");
 return pool;
 })
 .catch(err => console.log("Database Connection Failed! Bad Config: ", err));

module.exports = {
 mssql,
 commonConnPool
};

server.js

....

var commonConnPool= require('./commonDbConnection').commonConnPool;

(or)

var {commonConnPool} = require('./commonDbConnection')

....

//While setup CORS inject the DB connection pool and later you can retrieve it in the API library


app.use(async function (req, res, next) {
 res.header("Access-Control-Allow-Origin", "*");
 res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
 req.conn_pool = await commonConnectionPool;
 next();
});

...

app.use('/api', api);

anotherlibraryfile.js

var { commonConnPool, mssql } = require('./dbInitCommonConn');

async insertIntoDB(value1, value2, value3) {
 try {

await commonConnPool.then(pool => {
 pool
 .request()
 .input("param1", mssql.VarChar(50), value1)
 .input("param2", mssql.VarChar(50), value2)
 .input("param3", mssql.VarChar(50), JSON.stringify(value3)) //if its object, then stringify it before pass
 JSON.stringify(message)
 )
 .execute("usp_Insert_Record_SP");
 });
 } catch (err) {
 console.log("error occurred", err);
 }
 }